subtly (adv.)

c. 1300 (mid-13c. as a surname), sotil, "penetrating; ingenious; refined" (of the mind); "sophisticated, intricate, abstruse" (of arguments), from Old French sotil, soutil, subtil "adept, adroit; cunning, wise; detailed; well-crafted" (12c., Modern French subtil), from Latin subtilis "fine, thin, delicate, finely woven;" figuratively "precise, exact, accurate," in taste or judgment, "fine, keen," of style, "plain, simple, direct," from sub "under" (see sub-) + -tilis, from tela "web, net, warp of a fabric," from PIE root *teks- "to weave," also "to fabricate." According to Watkins, the notion is of the "thread passing under the warp" as the finest thread.

From early 14c. in reference to things, "of thin consistency;" in reference to craftsmen, "cunning, skilled, clever;" Depreciative sense "insidious, treacherously cunning; deceitful" is from mid-14c. Material senses of "not dense or viscous, light; pure; delicate, thin, slender; fine, consisting of small particles" are from late 14c. sotil wares were goods sold in powdered form or finely ground. Partially re-Latinized in spelling, and also by confusion with subtile.

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.