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.
early 15c., practicale "of or pertaining to matters of action, practice, or use; applied," with -al (1) + earlier practic (adj.) "dealing with practical matters, applied, not merely theoretical" (early 15c.) or practic (n.) "method, practice, use" (late 14c.).
In some cases directly from Old French practique (adj.) "fit for action," earlier pratique (13c.) and Medieval Latin practicalis, from Late Latin practicus "practical, active," from Greek praktikos "fit for action, fit for business; business-like, practical; active, effective, vigorous," from praktos "done; to be done," verbal adjective of prassein (Attic prattein) "to do, act, effect, accomplish; come to an end, succeed," literally "to pass through, travel," from PIE *per(h)- "go through, cross," an enlargement of the root *per- (2) "to lead, pass over."
Of persons, in reference to skills or occupations, "whose knowledge is derived from practice rather than theory," 1660s. The noun meaning "examination or lesson devoted to practice in a subject" is by 1934. Practical joke "trick played on someone for the sake of annoying him and raising a laugh at his expense" is from 1771 on the notion of "a jest carried into action" (earlier handicraft joke, 1741).