Middle English slouli, from Old English slawlice "sluggishly, slothfully, negligently;" see slow (adj.) + -ly (2). By late 14c. as "with slow motion or progress, not quickly or hastily."
Entries linking to slowly
slow adj.
Middle English slou, from Old English slaw "inactive by nature, sluggish, torpid, lazy, tardy in taking action," also "not clever, dull, not quick in comprehension," from Proto-Germanic *slæwaz (source also of Old Saxon sleu "blunt, dull," Middle Dutch slee, Dutch sleeuw "sour, tart, blunt," Old High German sleo "blunt, dull," Old Norse sljor, Danish sløv, Swedish slö "blunt, dull").
The meaning "taking a long time to develop, not happening in a short time" is attested from c. 1200; that of "not quick to move physically, taking a long time to move or go a short distance" is by mid-14c. The meaning "dull, tedious" is from 1841. Of clocks, from 1690s.
As an adverb c. 1500. As a noun from c.122, "the slothful, sluggards." The slows "imaginary disease to account for lethargy" is from 1843. Slow-paced "moving or advancing slowly" is from 1590s. Slow-motion (adj.) in cinematography is by 1929.
-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.