Old English stiward, stigweard "house guardian, housekeeper," from stig "hall, pen for cattle, part of a house" (see sty (n.1)) + weard "guard" (from Proto-Germanic *wardaz "guard," from PIE root *wer- (3) "perceive, watch out for").
Used after the Conquest as the equivalent of Old French seneschal (q.v.). Meaning "overseer of workmen" is attested from c. 1300. The sense of "officer on a ship in charge of provisions and meals" is first recorded mid-15c.; extended to trains 1906. This was the title of a class of high officers of the state in early England and Scotland, hence meaning "one who manages affairs of an estate on behalf of his employer" (late 14c.). Meaning "person who supervises arrangements" at a meeting, dinner, etc., is from 1703.
The Scottish form (with terminal -t attested from late 14c.) is reflected in Stewart, name of the royal house descended from Walter (the) Steward, who married (1315) Marjorie de Bruce, daughter of King Robert. Stuart is a French spelling, attested from 1429 and adopted by Mary, Queen of Scots.