'awnless' is formed in Modern English from the noun 'awn' and the suffix '-less' (from Old English 'lēas'), where '-less' meant 'without'.

'awn' was used in Middle English as 'aun' or 'aune' to refer to a bristle on a grain; the suffix '-less' comes from Old English 'lēas' and developed into the Modern English suffix '-less', so the compound became 'awnless' in Modern English.

Initially, the compound simply meant 'without an awn (a bristle on a grain)', and this basic meaning has remained in botanical usage to the present.