if a day

if (one's) a day

One is at least a particular age (which is typically specified right before this phrase is said). Stu's been with the company for almost 30 years, so I bet he's 50 if he's a day.
See also: if
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

if one's a day

Fig. a phrase attached to an expression of someone's age. She's fifty if she's a day! I'm sure he's forty-five if he's a day.
See also: if
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
See also:
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