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]

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "reefer":
   DET, DMT, Frigidaire, LSD, Mary Jane, Pullman, Pullman car, STP,
   THC, acid, antidepressant, ataractic, baggage car, boxcar, caboose,
   car, carriage, chair car, coach, coal car, covered waggon,
   day coach, diethyltryptamine, dimethyltryptamine, diner, dinghy,
   dining car, drawing room, electric refrigerator,
   electronic refrigerator, flat, flatcar, freezer ship, fridge, gage,
   ganja, gondola, grass, hallucinogen, hash, hashish, hay, hemp,
   ice chest, icebox, joint, kava, local, luggage van, mail car,
   mail van, marijuana, mescal, mescal bean, mescal button, mescaline,
   mind-altering drug, mind-blowing drug, mind-expanding drug,
   morning glory seeds, palace car, parlor car, passenger car, peyote,
   pot, psilocin, psilocybin, psychedelic, psychic energizer,
   psychoactive drug, psychochemical, psychotomimetic, railway car,
   refrigerator, refrigerator car, refrigerator truck, refrigeratory,
   roach, roomette, sleeper, smoker, smoking car, stick, stockcar,
   tank, tea, tender, tranquilizer, truck, van, waggon, weed