c. 1300, sengli, "individually, separately," from single (adj.) + -ly (2).
Entries linking to singly
single adj.
early 14c., "unmarried," from Old French sengle, sangle "alone, unaccompanied; simple, unadorned," from Latin singulus "one, one to each, individual, separate" (usually in plural singuli "one by one"), from PIE *semgolo‑, suffixed (diminutive?) form of root *sem- (1) "one; as one, together with."
The meaning "consisting of one unit, individual, unaccompanied by others" is from late 14c., often merely emphatic. The meaning "undivided" is from 1580s. Single-parent (adj.) is attested from 1966.
-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.