1520s, "but prob. in popular use from an earlier period" [OED], likely from tops, plural of top (n.1) "highest point" + obsolete terve "turn upside down, topple over," from Old English tearflian "to roll over, overturn," from Proto-Germanic *terbanan (cf. Old High German zerben "to turn round"). The Century Dictionary (1902) calls it "A word which, owing to its popular nature, its alliterative type, and to ignorance of its origin, leading to various perversions made to suggest some plausible origin, has undergone, besides the usual variations of spelling, extraordinary modifications of form." It lists 31 variations.
A high executive, officer, manager, etc: sighed one Pentagon topsider last week/ and also ''delightfully puzzled,'' as one topsider put it
[1960s+; related to late 1800s British topside, ''in control, commanding,'' in turn related to the nautical term topside by the suggestion that the master of a ship works on the bridge, which is the highest deck of the ship]