The Collaborative International Dictionary of English v.0.48:

Reefer \Reef"er\ (r[=e]f"[~e]r), n.
   1. (Naut.) One who reefs; -- a name often given to
      midshipmen. --Marryat.
      [1913 Webster]

   2. A close-fitting jacket or short coat of thick cloth.
      [1913 Webster]

   3. A marijuana cigarette. [Slang]
      [1913 Webster]

WordNet (r) 3.0 (2006):

reefer
    n 1: marijuana leaves rolled into a cigarette for smoking [syn:
         joint, marijuana cigarette, reefer, stick,
         spliff]