A daffynition (a portmanteau blend of daffy and definition) is a form of pun involving the reinterpretation of an existing word, on the basis that it sounds like another word (or group of words).
Presented in the form of dictionary definitions, they are similar to transpositional puns, but often much less complex and easier to create.
Under the name Uxbridge English Dictionary, making up daffynitions is a popular game on the BBC Radio 4 comedy quiz show I'm Sorry I Haven't a Clue.
A lesser-known subclass of daffynition is the goofinition, which relies strictly on literal associations and correct spellings, such as "lobster = a weak tennis player".
This play on words is similar to Cockney rhyming slang.