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.
1545, "not suitable" (in ref. to things), from un- (1) "not" + pp. of fit (see fit (adj.)). In ref. to persons or human qualities, attested from 1551. The verb meaning "to render unfit" is recorded from 1611.