A long partially erect penis
I slapped her ass cheek with my dondelinger after I came on her back
A playful term for 'penis', often used in a playful manner when a friend makes a silly comment. Taken from Principal Dondelinger from The Simpsons.
Person 1: "When does the 24-hour shop close?"
Person 2: "Oh my God, you're such a dondelinger!"