To quit a shady business or life; to reform and cut ties with wrongdoing—an idiom meaning to go straight.
彼は長年の裏稼業に別れを告げ、ついに足を洗った。
He said goodbye to his years in shady work and finally quit that life.
刑務所で更生し、元の仲間たちから足を洗うことにした。
After rehabilitating in prison, he decided to cut ties with his old gang.
親の勧めで彼女は足を洗い、新しい仕事を見つけた。
At her parents' urging she quit that life and found a new job.
Literally to wash one's feet (a literal, physical action).
砂まみれで帰ってきた子どもは庭で足を洗った。
The child, who came back covered in sand, washed his feet in the yard.
温泉に入る前に、まず足を洗ってください。
Before entering the hot spring, please wash your feet first.