A person who frowns or displays a frowning expression.
From 'frown' (Middle English, possibly from Old French 'frogne' meaning grimace) plus the agent suffix '-er', which creates a noun for someone who performs an action.
The suffix '-er' is one of English's most productive word-building tools—we can add it to almost any verb to create a person who does that action. 'Frower' is rare today, but it shows how flexible and creative English speakers were in forming new words.
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