To wipe one's penis around the edge of a cup, usually containining tea, before giving it to someone one does not like.
It is a term used largely in the British Army where new recruits or private soldiers are forced to brew up for lance corporals, corporals, sergeants etc... Skiffing the tea makes the tea brewer feel better about having to make the twat some tea in the first place.
"I hope you've not skiffed that tea Jones!"
"Jones, make four brews, unskiffed!"
"Did you skiff the corporal's tea?"
"Don't drink that I've just skiffed it."