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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".