Why was clemency trending last week?
The place where you put things when your pdl is full. If you don't have one and too many things get pushed, you forget something. The overflow pdl for a person's memory might be a memo pad. This usage inspired the following doggerel:
Hey, diddle, diddle The overflow pdl To get a little more stack; If that's not enough Then you lose it all, And have to pop all the way back. --The Great Quux
The term pdl seems to be primarily an MITism; outside MIT this term is replaced by "overflow stack".