"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.
Perhaps the original notion is from jeu de la poule, supposedly a game in which people threw things at a chicken and the player who hit it, won it, which speaks volumes about life in the Middle Ages. The notion behind the word, then, is "playing for money." The connection of "hen" and "stakes" is also present in Spanish polla and Walloon paie.
Meaning "collective stakes" in betting first recorded 1869; sense of "common reservoir of resources" is from 1917. Meaning "group of persons who share duties or skills" is from 1928. From 1933 as short for football pool in wagering. Pool shark is from 1898. The phrase dirty pool "underhanded or unsportsmanlike conduct," especially in politics (1951), seems to belong here now, but the phrase dirty pool of politics, with an image of pool (n.1) is recorded from 1871 and was in use early 20c.