1660s, "as regards a name," from nominal + -ly (2). Meaning "in name only" (as opposed to really) is attested from 1748.
Entries linking to nominally
nominal adj.
mid-15c., nominalle, "pertaining to nouns," from Latin nominalis "pertaining to a name or names," from nomen (genitive nominis) "name," which is cognate with Old English nama (from PIE root *no-men- "name"). Meaning "of the nature of names" (in distinction to things) is from 1610s. Meaning "being so in name only" is recorded from 1620s.
-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.
really adv.
c. 1400, "actually, in fact, in a real manner," originally in reference to the presence of Christ in the Eucharist, "substantially," from real (adj.) + -ly (2). The general sense is from early 15c. Purely emphatic use dates from c. 1600, "indeed," sometimes as a corroboration, sometimes as an expression of surprise or a term of protest; interrogative use (as in oh, really?) is recorded from 1815.