'gill-less' originates from the English word 'gill,' combined with the suffix '-less,' which means 'without.'

'gill' comes from the Old Norse word 'gil,' meaning 'ravine,' but in the context of fish anatomy, it refers to the respiratory organ. The suffix '-less' is from Old English 'lēas,' meaning 'devoid of.'

Initially, 'gill' referred to the respiratory organ of fish, and '-less' indicated absence. The combination 'gill-less' has consistently meant 'without gills.'