late 14c., "possible" (as opposed to actual), "capable of being or becoming," from Old French potenciel and directly from Medieval Latin potentialis "potential," from Latin potentia "power, might, force;" figuratively "political power, authority, influence," from potens "powerful," from potis "powerful, able, capable; possible;" of persons, "better, preferable; chief, principal; strongest, foremost," from PIE root *poti- "powerful; lord."
The noun, meaning "that which is possible, anything that may be" is attested by 1817 (Coleridge), from the adjective. Middle English had potencies (plural) "a caustic medicine" (early 15c.).
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/potentially">Etymology of potentially by etymonline</a>
Harper, D. (n.d.). Etymology of potentially. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/potentially