Kef refers to smoking material, such as hemp. It is also used to name the euphoria produced by smoking such material. This term is used in the Maghreb to refer to Indian hemp (Cannabis Indica). It probably gave rise to the stoner slang keef. The variant kif is a North African term for a mix of tobacco and marijuana.
In Arabic, kef means pleasure and well-being.
Kef is also writen kif, kayf, or kaif. Kef, kif, and kaif are excellent Scrabble words since they appear in even the third edition of The Official Scrabble Players Dictionary.
Sources: The Official Scrabble Players Dictionary, The American Heritage Dictionary, and http://www.deciderite.com/weedlair/pages/slang.html