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.
"small body of standing water," Old English pol "small body of water; deep, still place in a stream," from Proto-West Germanic *pōl- (source also of Old Frisian and Middle Low German pol, Dutch poel, Old High German pfuol, German Pfuhl "pool, puddle"), which is of uncertain origin, perhaps a substratum word [Boutkan]. As a short form of swimming pool it is recorded from 1901. Pool party "party at a swimming pool" is by 1965.