1
general::
Pay in full, discharge all that is owing, as in, It's late? let's pay up and go home. [c. 1800] Also see PAY OFF, def. 1.
American Heritage Idioms
2
general::
Phrase(s): pay up
to pay what is owed. (Often a command: Pay up!) • I want my money now. Pay up! • If you don’t pay up, I’ll take you to court., Phrase(s): pay something up
to pay all of whatever is due; to complete all the payments on something. • Would you pay up your bills, please? • Your dues are all paid up.
McGrawhill's American Idioms And Phrasal Verbs