fappenstance
A random occurence that results in the need or desire to start pleasuring oneself. A combination of fapping and happenstance.
I was on my way to the bathroom to take a piss when by fappenstance I walked in on my friend boning his girlfriend in the laundry room. Needless to say when I got to the bathroom I decided to pound off instead of piss.
Fappenstance
Something that happens by chance whilst fapping.
John was cranking down full throttle and Linda walked in by fappenstance; impressed by his tenacity she jumped in to ride his glory clutch.