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.
Verb(1): "Quit taffing around!"
Verb(2): "You mean he managed to fit -both- his hands up his gaping asshole?! You're taffing me!"
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.
"Aww, taff it all!"
"Quit taffing about, you!"
"What the taff was that?!"
"Are you taffing with me?"
"This shift has been completely taffed.."