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.
"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