v.1. To blunder because of mistakes or poor judgment: I've tried many times to pass this test, but this time I really fouled up.
2. To cause someone or something to blunder: The howling dogs distracted me and fouled up my concentration. The pain in my hand fouled me up and I couldn't aim my camera.
3. To clog or became entangled in something: The seaweed fouled up the propeller blades. The dangling cables fouled the machinery up, thereby causing the breakdown.