n. an excuse to take the easy way out, an excuse to not finish something in an acceptable way
This phrase was parodied by Monty Python
to end some sketches. Instead of a climax or punch line, the police (cops) would turn up and arrest the characters. Therefore, they would literally "cop out" of finishing the sketch. The most notable was at the end of "Monty Python and the Quest for the Holy Grail".