secondly adv.

c. 1300, "next in order, place, time, etc., after the first; an ordinal numeral; being one of two equal parts into which a whole is regarded as divided;" from Old French second, secont, and directly from Latin secundus "following, next in time or order," also "secondary, subordinate, inferior," from PIE *sekw-ondo-, pariticipal form of root *sekw- "to follow."

It replaced native other in this sense because of the ambiguity of the earlier word. From late 14c. as "other, another" (as in "No Second Troy"), also "next in order in rank, quality, or importance."

Second sight is from 1610s; it presumably implies a second way of seeing in addition to the physical sight with the eyes, but it is etymologically perverse as it means the sight of events before, not after, they occur or are revealed. Second-degree in a general sense of "next to lowest on a scale of four" in Arostotelian qualities is from Middle English; in reference to burns, by 1890. Second fiddle is attested by 1809:

A metaphor borrowed from a musical performer who plays the second or counter to one who plays the first or the "air." [Bartlett, "Dictionary of Americanisms," 1848]

Latin secundus, tertius, etc. appended to personal names in English schools (to designate boys having the same surname by order of seniority) is attested by 1826s.

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.