Inclined to frown; having a frowning expression or tendency; informal or colloquial for 'characterized by frowns'.
From 'frown' (Middle English, possibly from Old French 'frogne') with the informal suffix '-y' or '-ie', which turns nouns into casual adjectives (like 'sleepy', 'grumpy', 'dreamy').
The '-y' suffix is one of the most casual, childlike ways to turn a noun into an adjective in English—'frowny' feels playful and informal in a way that 'frownful' doesn't, showing how English has registers for different situations.
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