un·fit (n-ft) adj. 1. Not meant or adapted for a given purpose; inappropriate: a solvent that is unfit for use on wood surfaces. 2. Below the required standard; unqualified: an unfit parent. 3. Not in good physical or mental health. 4. Biology Unable to survive or produce viable offspring in a particular environment. tr.v. un·fit·ted, un·fit·ting, un·fits To cause to be unsuited or unqualified: “Having run for president . . . often unfits a man for lesser or more useful subsequent work” (Garry Wills).