1
general::
Stop acting or speaking foolishly or pretentiously, as in Oh come off it! you're no smarter than they are. This term, often used as an imperative, dates from the late 1800s, when it was usually put simply as come off
American Heritage Idioms
2
general::
Phrase(s): Come off it!
1. Inf. Stop acting arrogantly! (See also come off ((of)something).)) • Come off it, Tiff. You’re not the Queen of England. 2. Inf. Give up your incorrect point of view! • Come off it! You’re wrong, and you know it!
McGrawhill's American Idioms And Phrasal Verbs