A gay sex act performed in a public restroom lacking partitions between stalls, in which two seated participants take turns bending over sideways to perform fellatio on one another whilst simultaneously defecating in their respective receptacles. Generally not performed sequentially ("I'll do you until you blow out both your loads then you do me, ok?") but rather in rapid alternating bursts of a few seconds duration A-does-B then B-does-A. A professionally-executed blundonian blumpkin maneuver results in a virtually seamless and volcanic four-way explosion and is a true wonder to behold.
Conversation in public restroom:
Gay Guy #1: Are you thinking what I'm thinking?
Gay Guy #2: Blundonian blumpkin?
Gay Guy #1: Ha!