1
general::
1. Plead guilty or confess to a crime in exchange for a lighter sentence; also, plead guilty to a lesser charge in exchange for not being tried for a more serious charge. For example, Arnold decided he was better off copping a plea than facing a jury. [Colloquial; 1920s] 2. Plead for mercy; make excuses. For example, He copped a plea about not knowing his way around. [Slang; c. 1940]
American Heritage Idioms
2
general::
Phrase(s): cop a plea
Fig. to plead guilty to a lesser charge to avoid a more serious charge or lessen time of imprisonment. • He copped a plea and got off with only two months in the slammer.
McGrawhill's American Idioms And Phrasal Verbs