Английский Этимологический словарь - fool (n.)

- c.1200, from O.Fr. fol, from L. follis "bellows, leather bag," in V.L. used with a sense of "windbag, empty-headed person." Foolhardy is from M.E. hardi "bold;" foolproof is from 1902, Amer.Eng.; foolscap (pre-1700) was so called because this type of paper was originally watermarked with a court jester's cap. Fool around is 1875 in the sense of "pass time idly," 1970s in sense of "have sexual adventures."