"next in order after the first; an ordinal numeral; being one of two equal parts into which a whole is regarded as divided;" c. 1300, 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."
Replaced native other in this sense because of the ambiguity of the earlier word. Second sight is from 1610s; an etymologically perverse term, because it means in reality the sight of events before, not after, they occur. 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]