"so as not to be felt or perceived," early 15c.; see insensible + -ly (2).
Entries linking to insensibly
insensible adj.
c. 1400, "lacking the power to feel with the senses, numb, dazed" (now rare in this meaning), from Late Latin insensibilis "that cannot be felt," from in- "not" (see in- (1)) + sensibilis "having feeling: perceptible by the senses" (see sensible). Meaning "void of feeling, not susceptible to emotion or passion" is from 1610s. Meaning "incapable of being felt or perceived by the senses or the mind, so small or slight as to be imperceptible" is from late 14c. Compare insensate.
-ly 2
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.