late 13c., "to prepare, make preparations;" late 14c., "to equip, provide with proper clothing; dress or dress up," from Old French apareillier "prepare, make (someone) ready, dress (oneself)" (12c., Modern French appareiller), from Vulgar Latin *appariculare. This is either from Latin apparare "prepare, make ready" (see apparatus), or from Vulgar Latin *ad-particulare "to put things together," from Latin particula "little bit or part, grain, jot" (see particle (n.)). "The 15th c. spellings were almost endless" [OED].
By either derivation the sense is etymologically "to join like to like, to fit, to suit." Compare French habiller "to dress," originally "prepare, arrange," English dress, from Latin directus. The words were "specially applied to clothing as the necessary preparation for every kind action" [Wedgwood].
Cognate with Italian aparecchiare, Spanish aparejar, Portuguese aparelhar. Related: Appareled; apparelled; appareling; apparelling.