'unsmiling' originates from Modern English, specifically formed by the negative prefix 'un-' + the present participle 'smiling' of the verb 'smile', where 'un-' meant 'not' (from Old English) and 'smile' is the verb root.

'smile' comes from Old English/Middle English forms (such as Old English 'smīelan' / Middle English 'smilen'), which developed into the Modern English 'smile'; the prefix 'un-' derives from Old English 'un-' meaning 'not', and these combined in Modern English to form 'unsmiling'.

Initially, it meant simply 'not smiling', but over time it also came to describe a facial expression implying sternness, coldness, or lack of friendliness—i.e., 'stern or expressionless' as well as 'not smiling'.