A person who catches someone in a trap, either literally by setting a trap or figuratively by tricking them into a situation.
From 'entrap' (en- + trap) + the agent suffix -er. 'Trap' comes from Old Norse 'treppa' meaning to catch or snare. The 'en-' prefix means to put into or cause to be in that state.
Entrappers historically included animal trappers on the frontier, but the word gained a darker meaning during undercover police operations—when law enforcement officers deliberately set up situations to catch criminals, it raised serious questions about when catching lawbreakers becomes entrapment itself.
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