impracticable adj.

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.

1670s, "capable of being performed or affected," from French pratiquable (1590s), from pratiquer "to practice," from Medieval Latin practicare "to practice," from Late Latin practicus, ultimately from Greek (see practical). By 1710 as "capable of being actually used."

Possible notes that which may or might be performed if the necessary powers or means can or could be obtained ; practicable is limited to things which may he performed by the means that one possesses or can obtain. [Century Dictionary] 

1823, from assimilated form of in- (1) "not, opposite of" + practical (adj.). Impracticable in the same sense dates from 1670s; unpractical is rare. Related: Impractically.