To ejaculate onto a Mc Vitties Jaffa Cake, and then possibly give it to your sister so she can experience that salty pleasure
1. "Josh just Jaffed on that JaffaCake, he offered it to his sister." (He had cum all over the now soggy Jaffa)" 2."She loved the sight of him jaffing furiously last night"
A variation on the term "Effing and Blinding", meaning to swear or curse profusely. Used in the North of England, notably by one Karl Pilkington in an episode of the Ricky Gervais Show.
Karl: I've just slagged off thatoctopus, but at no point was I effing and jeffing about it.