early 15c., "narrow, drawn in, small," from Latin strictus "drawn together, close, tight," past participle of stringere (2) "to draw or bind tight" (see strain (v.)). The sense of "stringent and rigorous" (of law) is first found in 1570s; of qualities or conditions generally, 1590s.
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/strictly">Etymology of strictly by etymonline</a>
Harper, D. (n.d.). Etymology of strictly. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/strictly