mid-13c., "madly, foolishly, in a flighty or foolish manner," from giddy + -ly (2). Meaning "dizzily" is by 1729.
Entries linking to giddily
giddy adj.
Old English gidig, variant of gydig "insane, mad, stupid," perhaps literally "possessed (by a spirit)," if it is from Proto-Germanic *gud-iga- "possessed by a god," from *gudam "god" (see god (n.)) + *-ig "possessed." Meaning "having a confused, swimming sensation" is from 1560s (compare sense evolution of dizzy). Meaning "elated" is from 1540s. Related: Giddily; giddiness.
-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.