plural of po
"The court clerk stamped the document with a pos to confirm that all parties had received their copies."
Initialism of proof of service.
In plain English: A pos is slang for something that is cool, impressive, or high quality.
"He showed me all his new skateboards and other cool pos from downtown."
Clipping of positive.
"I'm going to get my morning coffee and a pos before heading to work."
Initialism of parent over shoulder (i.e. observing the young user's conversation)
"The teenager felt awkward when his parents stood behind him, giving him a pos while he texted his friends."