First attested from 1611. Compare with Dutch kolzwijn, kolsem, Low German, kielswîn, German Kielschwein, Danish kølsvin, kölsvin, all with the same meaning. First part is keel while the second part is uncertain; possibly sill.
keelson (plural keelsons)
1884, Dixon Kemp, A Manual of Yacht and Boat Sailing (Fourth Edition), page 13:
- With regard to materials, all the frames should be of oak and so should the stem piece, stern post, upper portion of dead woods, knight heads, apron, beams, shelf clamp, bilge strakes, and keelson; the keel will generally be found to be either English or American elm.