A unit of measure indicating the amount of Cannabis required for one joint - a "spliffs-worth". Which is a term not unlike the name of a butler.
The phrase originated in Bristol in the U.K. and has now penetrated the vocabulary of much of the U.K.'s Cannabis-smoking population. There are some reports indicating the term may be being used in the U.S.A.
"Excuse mate, any chance of getting a butler off you?"
Q"How much weed have you got left?"
A"A couple of Butlers"