adjective, -er, -est, adverb, noun, verb | 1. | grossly offensive to the senses; disgustingly loathsome; noisome: a foul smell. |
| 2. | containing or characterized by offensive or noisome matter: foul air; foul stagnant water. |
| 3. | filthy or dirty, as places, receptacles, clothes, etc. |
| 4. | muddy, as a road. |
| 5. | clogged or obstructed with foreign matter: a foul gas jet. |
| 6. | unfavorable or stormy: foul weather. |
| 7. | contrary, violent, or unfavorable, as the wind. |
| 8. | grossly offensive in a moral sense. |
| 9. | abominable, wicked, or vile, as deeds, crime, slander, etc. |
| 10. | scurrilous, profane, or obscene; offensive: foul language. |
| 11. | contrary to the rules or established usages, as of a sport or game; unfair: a foul blow. |
| 12. | Baseball. pertaining to a foul ball or a foul line. |
| 13. | limited in freedom of movement by obstruction, entanglement, etc.: a foul anchor. |
| 14. | abounding in errors or in marks of correction, as a printer's proof, manuscript, or the like. |
| 15. | Nautical.
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| 16. | North England and Scot.. not fair; ugly or unattractive. |
| 17. | Obsolete. disfigured. |
| 18. | in a foul manner; vilely; unfairly. |
| 19. | Baseball. into foul territory; so as to be foul: It looked like a homer when he hit it, but it went foul. |
| 20. | something that is foul. |
| 21. | a collision or entanglement: a foul between two racing sculls. |
| 22. | a violation of the rules of a sport or game: The referee called it a foul. |
| 23. | Baseball. foul ball. |
| 24. | to make foul; defile; soil. |
| 25. | to clog or obstruct, as a chimney or the bore of a gun. |
| 26. | to collide with. |
| 27. | to cause to become entangled or caught, as a rope. |
| 28. | to defile; dishonor; disgrace: His reputation had been fouled by unfounded accusations. |
| 29. | Nautical. (of barnacles, seaweed, etc.) to cling to (a hull) so as to encumber. |
| 30. | Baseball. to hit (a pitched ball) foul (often fol. by off or away): He fouled off two curves before being struck out on a fastball. |
| 31. | to become foul. |
| 32. | Nautical. to come into collision, as two boats. |
| 33. | to become entangled or clogged: The rope fouled. |
| 34. | Sports. to make a foul play; give a foul blow. |
| 35. | Baseball. to hit a foul ball. |
| 36. | foul out,
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| 37. | foul up, Informal. to cause confusion or disorder; bungle; spoil. |
| 38. | fall foul or afoul of,
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| 39. | foul one's nest. to dishonor one's own home, family, or the like. |
| 40. | run foul or afoul of, to come into collision or controversy with: to run foul of the press. |

foul one's nest
Also, foul one's own nest. Hurt one's own interests, as in With his constant complaints about his wife, he's only fouling his own nest. This metaphoric expression transfers a bird's soiling of its nest to human behavior. [Mid-1200s]