'mourn' originates from Old English, specifically the word 'murnian', where the root meant 'to grieve or be sorrowful'.

'murnian' changed into Middle English forms such as 'murnen' and 'mournen', influenced in part by Old French forms, and eventually became the modern English 'mourn'.

Initially, it meant 'to feel sorrow or grief', and over time it has retained that core meaning while extending to regret for non-person losses (e.g., institutions or eras).