The Transpositional pun reference article from the English Wikipedia on 24-Apr-2004
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Transpositional pun

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A transpositional pun is a complicated pun format with two aspects. Initially (like many other pun formats) it involves a clever redefinition of a well known word. The second aspect is that this redefinition occurs as the result of the transposition of the words in an otherwise well-known phrase or saying.

Some examples:

Alimony:The bounty of mutiny.
Hangover:The wrath of grapes.
Olympic Officials:The souls that time men's tries.
Saxophone:An ill wind nobody blows good.

As a result, transpositional puns are considered amongst the most difficult to create, and commonly the most challenging to comprehend, particularly for non-native speakers of English.