The Collaborative International Dictionary of English v.0.48:

deep-six \deep-six\ v. t.
   1. to discard in a deep body of water; -- also used
      figuratively, to discard contemptuously.

   Syn: deep six, give it the deep six.
        [WordNet 1.5]

   2. to throw from a boat into the water.

   Syn: deep six, throw overboard.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

deep-six
    v 1: toss out; get rid of; "deep-six these old souvenirs!" [syn:
         deep-six, give it the deep six]
    2: throw from a boat [syn: deep-six, throw overboard]