'motionless' originates from English, specifically the word 'motion' + the suffix '-less', where 'motion' ultimately comes from Latin 'motio' (from 'movere') meaning 'movement' and the suffix '-less' comes from Old English 'lēas' meaning 'without'.

'motion' passed into Middle English from Latin (via Old French), and the adjective-forming suffix '-less' from Old English combined with 'motion' to form 'motionless' in Early Modern English; the adverb 'motionlessly' is formed by adding the adverbial '-ly' to that adjective.

Initially it meant 'without movement' and this basic sense has remained stable; the derived adverb has kept the meaning 'in a manner lacking movement'.