1808, agent noun from twirl (v.). As baseball slang for "pitcher," by 1891.
Entries linking to twirler
twirl v.
1590s, "move round rapidly" (intransitive), of uncertain origin, possibly connected with Old English þwirl "a stirrer, handle of a churn," and Old Norse þvara "pot-sticker, stirrer." Or on another guess a blend of twist and whirl. Transitive sense, "cause to revolve rapidly," is from 1620s. Related: Twirled; twirling.