Approximately forty years old; around forty in age, or seeming to be in that age range.
From 'forty' plus the suffix '-ish', which comes from Old English and creates adjectives meaning 'somewhat, approximately, or having the quality of.' This is a productive suffix in modern English.
The '-ish' suffix is delightfully vague—'fortyish' lets you say someone's age without committing to exactness, which is why it's beloved in everyday conversation and why every language needs words that are deliberately fuzzy!
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