Fappinstance 

Interacting with someone who is masturbating, while you were already masturbating, either in person, via phone call, text, or internet.

I was wandering around jacking off when I walked in on Mike masturbating. He said it was a coincidence, but I think it was just fappinstance.

Fappinstance by Rynokie May 26, 2013

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 by MalibuMatt July 6, 2011

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.

Fappenstance by HotDeuceUniversity August 6, 2016