late 14c., "join or combine (with); attach (to something), adhere," from Old French aploiier "apply, use, attach" (12c., Modern French appliquer), from Latin applicare "attach to, join, connect;" figuratively, "devote (oneself) to, give attention," from ad "to" (see ad-) + plicare "fold" (from PIE root *plek- "to plait").
The etymological sense is "bring things in contact with one another." In English, from c. 1400 as "use or employ" something for a certain purpose;" from early 15c. of lotions, plasters, etc., "place in contact with the body," also, of one's mental powers or faculties, "put to work at a task or pursuit." Meaning "seek a job by submitting an application for one" is from 1851. A by-form applicate is recorded from 1530s. Related: Applied; applying.