Old English stiward, stigweard "house guardian," from stig "hall, pen" + weard "guard." 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.).
The Scottish form is reflected in Stewart, name of the royal house, from Walter (the) Steward, who married (1315) Marjorie de Bruce, daughter of King Robert. The terminal -t is a Scottish form (late 14c.). Stuart is a French spelling, attested from 1429 and adopted by Mary, Queen of Scots.