Etymology
Advertisement
foul (adj.)
Old English ful "rotten, unclean, vile, corrupt, offensive to the senses," from Proto-Germanic *fulaz (source also of Old Saxon and Old Frisian ful, Middle Dutch voul, Dutch vuil, Old High German fül, German faul, Gothic füls), from PIE *pu- (2) "to rot, decay," perhaps from the sound made in reaction to smelling something bad (see pus).

Old English ful occasionally meant "ugly" (as contrasted with fæger (adj.), modern fair (adj.)), and this sense became frequent in Middle English. The cognate in Swedish is the usual word for "ugly." Of weather from mid-14c. In the sporting sense of "irregular, unfair, contrary to established rule or practice" it is first attested 1797, though foul play is recorded from mid-15c. Baseball sense of "out of play" attested by 1860.
Related entries & more 
Advertisement
foul (v.)
Old English fulian "to become foul, rot, decay," from ful (see foul (adj.)). Transitive meaning "make foul, pollute" is from c. 1200. Meaning "become entangled" (chiefly nautical) is from 1832, probably from foul (adj.) in the sense "obstructed by anything fixed or attached" (late 15c.). "A term generally used in contrast to clear, and implies entangled, embarrassed or contrary to: e.g. to foul the helm, to find steerage impracticable owing to the rudder becoming entangled with rope or other gear" [Sir Geoffrey Callender, "Sea Passages," 1943]. Related: Fouled; fouling. Hence also foul anchor (1769), one with the slack of the cable twisted round the stock or a fluke; noted by 1832 as naval insignia.
Related entries & more 
foully (adv.)
Old English fullice; see foul (adj.) + -ly (2).
Related entries & more 
foulmart (n.)
"polecat," Middle English, from foul (adj.) + Old English mearþ "marten" (see marten).
Related entries & more 
befoul (v.)
"make foul, cover with filth," from Old English befylan; see be- + foul (v.). Related: Befouled; befouling.
Related entries & more 
Advertisement
foulness (n.)
Old English fulness "foulness, filthy smell;" see foul (adj.) + -ness. Similar formation in Old Frisian fulnisse, Dutch vuilnis, German fäulniss.
Related entries & more 
afoul (adv.)
1809, originally nautical, "in a state of collision or entanglement," from a- (1) + foul (adj.). From 1833 in general sense of "in violent or hostile conflict," mainly in phrases such as run afoul of.
Related entries & more 
foul-mouthed (adj.)
also foulmouthed, 1590s, apparently first in Shakespeare ["Henry IV," 1596]. Earlier were foul-tongued (1540s); foul-spoken (1580s).
Related entries & more 
filth (n.)
Old English fylð "uncleanness, impurity, foulness," from Proto-Germanic *fulitho (source also of Old Saxon fulitha "foulness, filth," Dutch vuilte, Old High German fulida), noun derivative of *fulo- "foul" (see foul (adj.)). A classic case of i-mutation.
Related entries & more 
fog (n.2)
"long grass, second growth of grass after mowing," late 14c., probably of Scandinavian origin; compare Norwegian fogg "long grass in a moist hollow," Icelandic fuki "rotten sea grass." A connection to fog (n.1) via a notion of long grass growing in moist dells of northern Europe is tempting but not proven. Watkins suggests derivation from PIE *pu- (2) "to rot, decay" (see foul (adj.)).
Related entries & more