microsecond (n.)

word-forming element meaning "small in size or extent, microscopic; magnifying;" in science indicating a unit one millionth of the unit it is prefixed to; from Latinized form of mikros, Attic form of Greek smikros "small, little, petty, trivial, slight," perhaps from PIE *smika, from root *smik- "small" (source also of Old High German smahi "littleness"), but Beekes thinks it a Pre-Greek word.

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.