'keelless' originates from English, specifically from the noun 'keel' plus the suffix '-less', where 'keel' meant the structural longitudinal piece of a ship and '-less' meant 'without'.

'keel' comes from Old English 'cēol' (related to a ship or keel) and is influenced by Old Norse 'kjǫlr'; the suffix '-less' comes from Old English '-lēas' (from Proto-Germanic *lausaz). The modern adjective 'keelless' is formed in English by combining these elements.

Initially the components meant 'keel' (structural/ship sense) and 'without'; over time the compound has maintained the literal meaning 'without a keel', now used both in nautical contexts and descriptively in biology.