'expressionlessly' originates from English, formed by adding the adverbial suffix '-ly' to 'expressionless', which itself is 'expression' + the suffix '-less'. 'Expression' comes from Latin 'expressio' (from 'exprimere'), where 'ex-' meant 'out' and 'primere/primere' (from 'primere' variant 'primere'/'primere') meant 'to press'.

'expression' entered English via Old French 'expression' from Latin 'expressio'; 'expressionless' developed in post‑Middle English by combining 'expression' + '-less', and 'expressionlessly' is the later adverbial formation adding '-ly' to the adjective 'expressionless'.

Initially, 'expression' referred to the act of pressing out or an outward representation; over time it came to mean an outward sign or display of feeling. 'Expressionless' and 'expressionlessly' have meant 'without outward sign or feeling' since their formation.