'mirthless' originates from Old English, specifically the word 'mirth' (Old English 'myrgð' / 'myrth') combined with the suffix '-less' from Old English 'lēas', where 'mirth' meant 'joy' and 'lēas' meant 'free from'.

'mirth' changed from Old English 'myrgð' (or forms such as 'myrth') into Middle English 'mirthe' / 'mirth', and combined with the Old English suffix '-lēas' to form Middle English 'mirthless', which eventually became the modern English word 'mirthless'.

Initially, it meant 'without joy' or 'devoid of mirth', and this basic meaning has largely remained; over time it also gained the more specific sense of describing a laugh or smile that is insincere or bitter.