duff
n. an inert substance used to dilute a narcotic such as cocaine.  

The substance must be similar in appearance to the narcotic so as to satisfy the suspicions of the narcotic purchaser.  The introduction of 'duff' to a narcotic increases the profit margin for the seller if the selling price of the product is not adjusted for the amount of duff introduced.

See also cut
"This shit isn't anything but duff!"  A phrase one may use when the narcotic purchased for one's pleasure does not administer the desired effect. 