Foul . An infraction or violation of the rules of play.
If you thought fouls were limited to the football field, you thought wrong.
Fouling goes from dogs and dirtying (see above) to a politician cheating ("foul play").
Someone in a foul temper becomes angry or violent very suddenly and easily. Weather can be foul too, when it's stormy, windy, etc. - it's often used in the UK :-) And a foul smell is a disgusting one!
Foul play. Cheating; unfair or illegal actions or bad practice.
Fall foul of someone. You do something which gets you into trouble with them.
Foul something up. Make a mistake and ruin something/ irreparable mis-management.