keel·son (kēl'sən, kěl'-) n.
Nautical A timber or girder fastened above and parallel to the keel of a ship or boat for additional strength.
[Alteration (influenced by keel1) of Middle English kelswin, probably from Old Norse *kjölsvīn : kjölr, keel + svīn, swine, timber; see sū- in Indo-European roots.]