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To touch yourself or someone else in a way that could be deemed innapropriate sexually.
"Watch out for that Neil bloke he's a bit of a cock feffing rapist!"

"His hand were in his pockets and he was clearly feffing himself."
by Boneyemski September 30, 2009

Can also be used as Feff meaning Fuck, or Feffer meaning Fucker.

- Originated from Effing
Ashley's a Feffing fag!
Stop being such a Feffer!
by Shalinet February 21, 2008