'armlessly' originates from English, specifically the word 'armless', where 'arm' meant 'limb' and the suffix '-less' meant 'without', and the adverbial suffix '-ly' formed the adverb.

'arm' comes from Old English 'earm' (from Proto-Germanic *armuz), Middle English used 'arm' for the limb; adding the suffix '-less' in Middle English produced 'armless', and later the suffix '-ly' produced 'armlessly' in Modern English.

Initially it meant 'without an arm' (literally lacking an arm or arms); over time it has retained that literal sense and is used chiefly in descriptive or artistic contexts.