mid-15c., a child's toy, from whirl (v.) + gig (see gig (n.1)). Meaning "anything in constant motion" is from 1580s; "fickle, flighty person" is from c. 1600; as a type of water beetle, from 1713.
Entries linking to whirligig
whirl v.
c. 1300, probably from Old Norse hvirfla "to go round, spin," related to hvirfill "circle, ring, crown," and to Old English hweorfan "to turn" (see wharf). Related: Whirled; whirling. Whirlybird "helicopter" is from 1951.
gig n.1
"light, two-wheeled carriage, usually drawn by one horse" (1791), also "small boat," 1790, perhaps imitative of bouncing. There was a Middle English ghyg "spinning top" (in whyrlegyg, mid-15c.), also "giddy girl" (early 13c., also giglet), from Old Norse geiga "turn sideways," or Danish gig "spinning top." Similar to words in continental Germanic for "fiddle" (such as German Geige); the connecting sense might be "rapid or whirling motion."