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 nowsoggy 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 that octopus, but at no point was I effing and jeffing about it.
And dying in yo' bed many years from now, would ya be willing ta trade all da days from dis here day ta dat fo' one chance, just one chance, ta come back here as young men an' tell our enemies dat dey may take our lives but dey will never take our freedom ... YUB NUB!