"A Pound of Flesh": This phrase comes from Shakespeare’s Merchant of Venice. In this play, Shylock, a moneylender, agrees to finance a fleet of ships for a young merchant, Antonio.  In the contract, Shylock demands a pound of flesh as payment should anything happen to the ships.  
 
 When the ships are lost at sea, Shylock insists that he must have a pound of flesh as the contract demanded.  —Antonio is spared only because of technicality: the contract did not say Shylock was entitled to any of Antonio’s blood. Thus, he cannot take a pound of Antonio’s flesh unless he can do so bloodlessly -- an impossibility. 

This phrase is used to describe someone’s insistence on being repaid, even if the repayment will destroy or harm the debtor. Example: Sure, that initial low rate for a credit card is tempting, but the credit card company will want their pound of flesh when you get over your head in debt.

—According to legend, Gordius was a Greek king.  He tied an extremely complex knot, and an oracle prophesied that whoever untied it would rule all of Asia. 
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—Alexander the Great “untied” the knot simply by cutting through it with a sword.

—A “Gordian Knot” is any extremely complex problem, and “cutting the Gordian Knot” refers to solving such a problem in a quick decisive manner.
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—Example: Each year the school counselors face the Gordian knot of scheduling classes.
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—Example: The councilwoman cut the Gordian knot so that two worthy programs would not have to be cut.  Cutting both programs budgets prevented from having to eliminate one or the other.

—In the Gospels, Judas Iscariot was the disciple who agreed to betray Jesus to the authorities in exchange for payment. The thirty pieces of silver were the price of Judas’s betrayal. The way Judas identified Jesus for the authorities was to approach and greet him with a kiss of identification. 
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“Thirty pieces of silver” refers to payment received for an act of treachery.

—“Betrayed with a kiss” refers to a supposed friend’s treachery.
 
Example: Patrick Henry warned his listeners about the supposed friendliness of the British.  He warned, “Suffer not yourselves to be betrayed with a kiss.”

Example: When the senator abruptly changed his vote to support the wealthy developer’s project, some of his colleagues muttered that he would probably receive his thirty pieces of silver from the developer. 

—In the Sermon on the Mount, Jesus admonished his followers to “cast not your pearls before swine.”  
 
—That is, his followers were to deliver their message to those who would appreciate it, not to those incapable of appreciating something of value.  Swine, or pigs, would be unable to appreciate pearls if the jewels were given to them.

—To “cast one’s pearls before swine” is to offer something precious to someone, or a group of people, unable to appreciate the value of what they are being given.
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Example: When Mr. Whitman makes a profound point that we students just do not understand, he sometimes shakes his head and mutters, “Pearls before swine.”

—A French poet, Alfred de Vigny, was said to have shut himself in an ivory tower so that he could compose his poems.  Furthermore “ivory towers” are mentioned in various 18th-century fairy tales. The term thus refers to a beautiful, unreachable place. It has come to have negative connotations of being out of touch with reality.

—A person who is secluded or protected from the real world and thus out of touch with reality is said to be residing in an “ivory tower.” Professors in academia are often said to being living "in their ivory towers."

Example: The engineers feared that the leaders of the company, sitting in their ivory tower,
did not understand why the people in the field needed increased resources to insure the new bridge truly would be safe.

—In Greek mythology, Sirens were sea creatures who lured sailors to their deaths on the rocky shores by singing a beautiful, irresistible song.  They are usually depicted as half-woman, half-bird.

In modern usage, “sirens” can refer to anything that tempts a person away from safety and toward a destructive path. A “siren song” is the temptation used to lure a person.
 
—Example: In his Speech in the Virginia Convention, Patrick Henry urged his listeners not to be fooled by an “illusion of hope,” saying, “We are apt to shut our eyes against a painful truth, and listen to the song of that siren, till she transforms us into beasts.” He is comparing false hope to the sirens and to Circe, who turned Odysseus’ men into swine.

—In Speed the Plough, a 1798 play by Thomas Morton, Mrs. Grundy is a character who never appears on stage.
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—However, other characters frequently ask, “What would Mrs. Grundy say?” Mrs. Grundy is a narrow-minded, conventional, prudish person.

—The word “Grundyism” and the phrase “Mrs. Grundy” refer to such an attitude of narrow-minded prudishness.
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Example: My mother said, “At the risk of being a Mrs. Grundy, I really don’t think you should go out in public in that outfit.”

The word albatross is sometimes used metaphorically to mean a psychological burden that feels like a curse.

It is an allusion to Samuel Taylor Coleridge's poem The Rime of the Ancient Mariner (1798) -- which, in part, inspired Mary Shelley's Frankenstein. In the poem, an albatross starts to follow a ship — being followed by an albatross was generally considered an omen of good luck. However, the mariner shoots the albatross with a crossbow, which is regarded as an act that will curse the ship (which indeed suffers terrible mishaps). To punish him, his companions induce him to wear the dead albatross around his neck indefinitely (until they all die from the curse). Thus the albatross can be both an omen of good or bad luck, as well as a metaphor for a burden to be carried as penance.

The symbolism used in the Coleridge poem is its highlight. For example:
 
Ah ! well a-day ! what evil looks
Had I from old and young !
Instead of the cross, the Albatross
About my neck was hung.

An "albatross around one's neck" means a burden which some unfortunate person has to carry.

According to Greek Mythology, when Achilles was a baby, his mother dipped him in the River Styx because the waters from this river gave immortality to humans.  His mother held him by his heel, so that was the only place on his body not touched by the water. From them on, Achilles’ heel was his one area of vulnerability.  Eventually, Achilles was killed during the Trojan War when a poisoned arrow struck his heel.  

Today, the term has come to refer to a person’s area of particular vulnerability.  

Examples: 
Maria's inability to resist rich desserts was her Achilles’ heel, keeping her from losing the ten pounds she wanted to lose. 
 
Senator Cianci's desire to be liked by everyone was his Achilles’ heel, preventing him from taking a strong stand on any issue and leading to his eventual defeat in the elections.