staunchly (adv.)

early 15c., "impervious to water," from Old French estanche "firm, watertight," fem. of estanc "tired, exhausted, wearied, vanquished; water-tight; withered, dried" (Modern French étanche), from Vulgar Latin *stanticare (source also of Spanish estanco "water-tight," Italian stanco "exhausted, weary"), probably from Latin stans (genitive stantis), present participle of stare "to stand," from PIE root *sta- "to stand, make or be firm." Sense of "strong, substantial" first recorded mid-15c.; of persons, "standing firm and true to one's principles" from 1620s.

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.