1
general::
Stop working or abandon an activity, as in Let's pack it in for the day. This usage alludes to packing one's things before departing, and during World War I became military slang for being killed. It Also is used as an imperative ordering someone to stop, as in Pack it in! I've heard enough out of you. In Britain it is Also put as pack it up. [Colloquial; early 1900s]
American Heritage Idioms
2
general::
Phrase(s): pack it in
1. Fig. to quit trying to do something; to give up trying something and quit. • I was so distressed that I almost packed it in. • I’ve had enough! I’m going to pack it in. 2. Fig. to go to bed. • Good night. It’s time for me to pack it in. • We drove to a hotel and packed it in.
McGrawhill's American Idioms And Phrasal Verbs