pool (n.1)
"small body of water," Old English pol "small body of water; deep, still place in a river," from Proto-West Germanic *pol- (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. As a short form of swimming pool it is recorded from 1901. Pool party is from 1965.