Is it farther or further?
"sullen," 1744, probably from Old English asolcen "idle, lazy, slow," from past participle of aseolcan "become sluggish, be weak or idle" (related to besylcan "be languid"), from Proto-Germanic *seklanan (cf. Middle High German selken "to drop, fall").
"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.
originally a light, open, one-horse, four-wheeled vehicle with its single seat for only one person fixed on its shafts. It is thought to have been invented in the early 19th century by an English physician and was supposedly named for his sulkiness in wishing to sit alone. The sulky was adapted to two wheels and widely used in the United States by doctors and others who had to travel extensively by themselves.