The Collaborative International Dictionary of English v.0.48:

lav \lav\ n. [by truncation of lavatory.]
   Same as lavatory[5]; -- a shortened form of the word.
   [informal]

   Syn: toilet, lavatory, can, facility, john, privy, bathroom.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

lav
    n 1: a room or building equipped with one or more toilets [syn:
         toilet, lavatory, lav, can, john, privy,
         bathroom]