monkey trap

plural monkey traps

1. literallyA cage containing a banana with a hole large enough for a monkey's hand to fit in, but not large enough for a monkey's fist (clutching a banana) to come out; anecdotally used to catch monkeys that lack the intellect to let go of the banana and run away.
2. figurativelyA clever trap of any sort, that owes its success to the ineptitude or gullibility of the victim.

   Synonyms: monkey jar