The Collaborative International Dictionary of English v.0.48:

Drying \Dry"ing\, a.
   1. Adapted or tending to exhaust moisture; as, a drying wind
      or day; a drying room.
      [1913 Webster]

   2. Having the quality of rapidly becoming dry.
      [1913 Webster]

   Drying oil, an oil which, either naturally or after boiling
      with oxide of lead, absorbs oxygen from the air and dries
      up rapidly. Drying oils are used as the bases of many
      paints and varnishes.
      [1913 Webster]

WordNet (r) 3.0 (2006):

drying oil
    n 1: an oil that hardens in air due to oxidation and is often
         used as a paint or varnish base