To be in a necessary state of hauling ass out of a particular location due to a deadline or other time-sensitive commitment, typically despite the ass-hauler's desire to stay longer. This idiom references Cinderella's midnight dash out of Prince Charming's caste. It also verbs the word pumpkin, conflating the "in" ending for the colloquial pronunciation of the gerundive "ing."

Becoming sleepy and needing bedtime visibly, especially when talking about events that might occur past your bedtime.

From Cinderella's "turning into a pumpkin" at a certain hour, here we have events that certain people generally don't get to because it'll interfere with their bedtime.

When you are doing stuff, but it's midnight and you are about to expire (like Cinderella).

Slang

Adj. (pmpkn, pm-, png-)
'p&m(p)-k&n

1) Excellent; first-rate

2) Very attractive physically, etc

3) Marked by a lack of intelligence or care; foolish or careless

4) A lot of, existing in great quantity or ample supply. Enough to fill the orange vine fruit "Pumpkin", that is widely cultivated in early autumn.

A word used for an adorable person, or one that you love.

The act of driving around neighboorhoods looking for pumpkins in yards, snatching that pumpkin, driving away with the pumpkin, and then throwing that pumpkin at things from the moving vehicle (i.e. mailboxes, cars, lawn ornaments)

A term of address to a little child.