From The Jargon File (version 4.4.7, 29 Dec 2003):
pop
/pop/
[from the operation that removes the top of a stack, and the fact that
procedure return addresses are usually saved on the stack] (also
capitalized ‘POP’)
1. vt. To remove something from a stack. If a person says he/she has
popped something from his stack, that means he/she has finally finished
working on it and can now remove it from the list of things hanging
overhead.
2. When a discussion gets to a level of detail so deep that the main point
of the discussion is being lost, someone will shout “Pop!”, meaning “Get
back up to a higher level!” The shout is frequently accompanied by an
upthrust arm with a finger pointing to the ceiling.
3. [all-caps, as ‘POP’] Point of Presence, a bank of dial-in lines allowing
customers to make (local) calls into an ISP. This is borderline techspeak.