if he is a day
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if she/he is a day
Used to emphasize that someone must be a certain age. How does he keep himself looking so young—he's got to be 60 if he's a day!
if he (or she) is a dayat least (added to a statement about the age of a person or thing).
1992 Shashi Tharoor Show Business Lawrence must be fifty if he's a day.