"quietly sullen," 1744, of uncertain origin. Connection has been suggested to obsolete, rare sulke "hard to sell" (1630s) and to Old English asolcen "idle, lazy, slow," past participle adjective from aseolcan "become sluggish, be weak or idle" (related to besylcan "be languid"), from Proto-Germanic *seklan (cognates: Middle High German selken "to drop, fall"). But words of similar meaning often are held to be imitative (compare miff, mope, boudoir). Related: Sulkily; sulkiness.

"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 and obliges the rider to be alone.