If something goes afoul, it goes badly wrong. When a prank falls afoul of the law, it crosses the line from just a joke to something much more serious.

Use the adjective afoul to describe things that conflict or clash. If a politician goes afoul of the rules about using taxpayer money for person items, she's bound to pay for it in votes. Likewise, a party guest might accidentally go afoul of the etiquette about how much candy to grab from a fallen piƱata. The original nineteenth century meaning of afoul was nautical, describing tangled ropes or lines on a ship.

Definitions of afoul

adj especially of a ship's lines etc

“with its sails afoul
foul, fouled
in a confused mass

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