Chelm:

In jewish floklore and legend, Chelm is a little village in Eastern Europe inhabited by amiable simpletons.  It has been told that the angel carrying the bag of the souls of the stupid for world distribution accidently dropped it.  They all spilled out in one place:  Chelm.

Chelmniks are unwitting masters of the nonsequitor.  It is not so much that they are irrational or illogical but they seem to live in a world not governed by logic. Leo Rosten in "The Joys of Yinglish" writes that the Chelm is the equivalent of Holland's "Kampen",  Italy's "Cuneo", and Germany's 'Schildburg'--all famous for boneheads, dunces and nincompoops".   Add to that list the village of Gotham in England, where the wise men were all idiots.

The following are Chelmisms, for which we are indebted to Leo Rosten:
a. Sleep faster, we need the pillows.
b. In Chelm, the inhabitants go to a dentist to have wisdom teeth put in.
c. In Chelm, the best cobbler, among all the tailors, is Chaim Yudel, the baker.

Chelm, ofcourse, stands for a place where fools dwell.

Catch-22:

Wildly satirical novel by the American writer Joseph Heller (1923-1999).  The protagonist, Yossarian, is a bombardier in an American squadron stationed on a Mediterranean island during World War II.  His commander is a fanatic bent on sacrificing his men to his own ambitions. In desperate efforts to survive, Yossarian feigns madness.  But, argues the military bureaucracy, referring to regulation 22, how can one be mad if one is trying to get out of the war?  There's the catch---Catch -22!

"Catch-22" is widely used to mean a paradoxical bind out of which one cannot work one's way, an absurd dilema.

caveat emptor:

Latin for "let the buyer beware".  In other words, if the buyer gets something that is inferior or defective, he has no one to blame but himself.  "Caveat Emptor" tells the buyer he should try to be informed and intelligent about what he buys.  It does not (as it appears) give the seller a free hand to cheat the buyer.

The complete quotation, however, puts some responsibility on the buyer. 
"Caveat emptor, quia ignorare non debuit quod ius alienum emit" (Let a purchaser beware, for he ought not to be ignorant of the nature of the property that he is buying for another party)

By natural extension, the "caveat" applies to a person who "buys" ideas, principles or codes of behavior.  He needs to be as careful and as informed as if he were buying goods of any kind.

~Facts on File Dictionary of Cultural and Historical Allusions
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