1540s, "not suitable" (in reference to things), from un- (1) "not" + fit (adj.). Related: Unfitness. In reference to persons or human qualities, attested from 1550s.
unfit (v.)
"to render unfit," 1610s, from unfit (adj.), or else from un- (2) "reverse of" + fit (v.). Related: Unfitted; unfitting.
unfit for human consumption
an unfit parent
certified as unfit for army service
drunk and unfit for service
fat and very unfit
unfeeling
unfeigned
unfelt
unfetter
unfinished
unfit
unflagging
unflappable
unflattering
unfledged
unflinching
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