Origin and history of impartial

late 14c., "not whole or total, incomplete;" early 15c., "one-sided, biased, inclined to favor one party in a cause or one side of a question more than the other," also "pertaining to a selfish interest rather than to a common or larger good," from Old French parcial (14c., Modern French partial) and directly from Medieval Latin partialis "divisible, solitary, partial," from Latin pars (genitive partis) "a part, piece, a share, a division" (from PIE root *pere- (2) "to grant, allot").

Weakened sense of "favorably disposed" is from 1580s. Meaning "affecting a part only, not universal or general" is by 1640s.

"fairness, freedom from bias," 1610s; see impartial + -ity.

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.