late 14c., "stated explicitly, not implied, clearly made known" from Old French espres, expres (13c.), from Latin expressus "clearly presented, distinct, articulated precisely," past participle of exprimere (see express (v.1)). Also late 14c. as an adverb, "specially, on purpose;" it also doubled as an adverb in Old French. An express train (1841) originally was one that ran to a certain station.
common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).
Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.
<a href="https://www.etymonline.com/word/expressly">Etymology of expressly by etymonline</a>
Harper, D. (n.d.). Etymology of expressly. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/expressly