Biology. pertaining to an organism or population that is not adapted to prevailing conditions or is not producing offspring in sufficient numbers to maintain its contribution to the gene pool of the next generation.
1540s, "not suitable" (in reference to things), from un- (1) "not" + past participle of fit (see fit (adj.)). Related: Unfitness. In reference to persons or human qualities, attested from 1550s. The verb meaning "to render unfit" is recorded from 1610s.