The Collaborative International Dictionary of English v.0.48:

Irrigate \Ir"ri*gate\, v. t. [imp. & p. p. Irrigated; p. pr. &
   vb. n. Irrigating.] [L. irrigatus, p. p. of irrigare to
   irrigate: ir- in + rigare to water; prob. akin to E. rain.
   See Rain.]
   [1913 Webster]
   1. To water; to wet; to moisten with running or dropping
      water; to bedew.
      [1913 Webster]

   2. (Agric.) To water, as land, by causing a stream to flow
      upon, over, or through it, as in artificial channels.
      [1913 Webster]

   3. (Med.) To rinse (a wound, infected area, etc.) with a flow
      or spray of a liquid.
      [PJC]

WordNet (r) 3.0 (2006):

irrigate
    v 1: supply with water, as with channels or ditches or streams;
         "Water the fields" [syn: water, irrigate]
    2: supply with a constant flow or sprinkling of some liquid, for
       the purpose of cooling, cleansing, or disinfecting; "irrigate
       the wound"