As mentioned previously, it is a word invented by Looking Glass Studios and originally saw its introduction in Thief: The Dark Project.
However, what the previous poster failed to note, is its additional use in later releases as "an expletive sounding of British origin." Quite frankly, only one word in the modern vernacular is versatile enough to take its place: The F Word.
See examples, below.
"Where did that taffer go?"
"Aww, taff it all!"
"Quit taffing about, you!"
"What the taff was that?!"
"Are you taffing with me?"
"This shift has been completely taffed.."
A word invented by the now-defunct Looking Glass Studios, used as a slang term in their 'First-Person Sneaker' release, Thief: The Dark Project (So named because of the games engine, which is simply called 'Dark' because of it's innovative use of light and darkness in gameplay.)
Taffer, insofar as the game environment is defined--is a bastardization of the word 'Trickster', a pagan deity with druidic tendencies. Much in the same way that the interjection 'Geez' in our English language is a bastardization of 'Jesus', the Christo-Catholic Messiah.
No direct denotation is given to the word 'Taffer', but from it's use, we can take it to be synonymous with many different nouns depending on how it's used. It can also be used as a verb:
Thief, Fool, Jerk, Joker, etc.
Noun(1): "Give that back this instant, you taffer!"
Verb(1): "Quit taffing around!"
Verb(2): "You mean he managed to fit -both- his hands up his gaping asshole?! You're taffing me!"