Halfwit

[half-wit, hahf-] /ˈhælfˌwɪt, ˈhɑf-/

noun
1.
a person who is feeble-minded.
2.
a person who is foolish or senseless; dunderhead.
/ˈhɑːfˌwɪt/
noun
1.
a feeble-minded person
2.
a foolish or inane person
n.

1670s, originally “a would-be wit whose abilities are mediocre;” sense of “simpleton” (one lacking all his wits) is first attested 1755.

Half-wits are fleas; so little and so light,
We scarce could know they live, but that they bite.
[Dryden, “All for Love”]

Phrase out of half wit “half out of one’s mind” was in Middle English (late 14c.). Half-witted “lacking common sense” is from 1640s.

noun

A foolish person: half-wits in the phys ed class (1755+)