Definition -
(1) An ambiguously structured clause or sentence that can be interpretted in multiple ways.
(2) A figure of speech where the ambiguity is purposely created by using an ambiguously structured clause or sentence.

Example -
"I once shot an elephant in my pajamas. How he got in my pajamas I'll never know."
(Groucho Marx, Animal Crackers)

Etymology -
The word derives from the Greek amphibolia, ambiguity.

Oxford English Dictionary -
Its first citation for sense (1) is from circa 1610:
"What a crafty Amphibolie or Æquivocation."
(Holland Camden's Brit. i. 307)

Its first citation for sense (2) is from 1588:
"Amphiboly, when the sentence may bee turned both the wayes, so that a man shall be uncertayne what waye to take."
(Fraunce Lawiers Log. i. iv. 27b,)

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