—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.”