'asymmetrical-gilled' is a compound formed from 'asymmetrical' and 'gilled'. 'asymmetrical' originates from Greek 'asymmetros', where 'a-' meant 'not' and 'symmetros' meant 'measured'; 'gilled' is derived from the noun 'gill' referring to a respiratory or lamellar structure.

'asymmetrical' entered English via late Latin/Greek formations and Middle English usage, and in modern English it combined with the noun-derived adjective 'gilled' to form the descriptive compound 'asymmetrical-gilled'.

Initially 'asymmetrical' meant 'not measured or not symmetric' in a geometric sense; over time it broadened to describe general lack of symmetry or evenness, which is preserved in the compound meaning 'having uneven gills'.