if he's, she's, etc. a day
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!
ˌif he’s, she’s, etc. a ˈday(informal) (used when talking about somebody’s age) at least: She isn’t forty! She’s fifty-five if she’s a day!
See also: if