The word “Foul” means;

a : Offensive to the senses : loathsome–Foul langauge.
b : filled or covered with offensive matter, being in an environment containing immoral energies, which causes a negative body chemical reaction that leads to anger, insult or violence.

: full of dirt or mud

a : immoral or spiritually odious : that which arouses or stimulates hate and incites the negative traits of irritation, and also choreographs profanity. 
detestable <a foul crime>