Some time during the 1930s, the phrase “cop out” became slang for pleading guilty, especially to a lesser charge as the result of plea bargaining. But the big change came in the 1950s when to “cop out” meant the individual made a full confession of some crime or misdemeanour. This was also known as “copping a plea” and usually, but not necessarily, involved confessing to the police.
It didn’t take long before it meant backing down or surrendering, especially a criminal or unconventional lifestyle. By the 1960s, the phrase “cop out” morphed into meaning a person who sidestepped issues, avoided fulfilling a duty or promise, or refused to fulfill expectations despite previous assertions to the contrary. This was achieved by making excuses or taking the easy way out, usually by finding some somewhat believable pretext that excused the individual from a situation.
In other words, “copping out” became slang for refusing to shoulder responsibilities in an attempt to avoid real or perceived personal or professional troubles.
Historically Speaking 