run foul of (someone or something)

1. nautical To collide or become entangled with something. The schooner lost control and ran foul of the lead boat. The small power boat ran foul of the seaweed and was completely immobilized.

2. To be in severe disagreement, trouble, or difficulty with someone or something; to be at odds with someone or something, especially due to disobeying rules or laws. Always look into the laws of any place you visit, or you may end up unwittingly running foul of the local police. Ms. Banks has run foul of this university for the last time. She is no longer welcome here!

run foul of

come into conflict with; go against.

This expression is nautical in origin: when used of a ship it means ‘collide or become entangled with an obstacle or another vessel’. Both literal and figurative uses were current by the late 17th century.