[Perhaps alteration of obsolete sulke, sluggish, perhaps ultimately from Old English āsolcen, from past participle of āseolcan, to become sluggish.] sulk'i·ly adv., sulk'i·ness n.
"light carriage with two wheels," 1756, apparently a noun use of sulky (adj.), on notion of "standoffishness," because the carriage has room for only one person.