[or more foul; most foul]
1 : very unpleasant to taste or smell
the foul odor of rotten eggs
The medicine left a foul taste in my mouth.
a foul-smelling chemical
2 : morally bad : very evil
a foul crime
3 : very bad or unpleasant
He was in a foul [=angry] mood.
The weather has been foul all week.
They do their job in fair weather and foul. [=in good weather and bad weather]