late 14c., from O.Fr. aplier (Mod.Fr. appliquer), from L. applicare "to attach to, to devote oneself to," from ad- "to" + plicare "fold" see ply (v.)). The etymological sense is "to bring things in contact with one another."
"put to practical use," (as opposed to abstract or theoretical), 1650s, from p.p. of apply (q.v.).