Help with the meaning plz ( She's 50 if sh's a day)

[moonlike]

Hi
Could you help me whit the meaning of the following sentence. I cam across it in a book. There's not enough context. It was in a list of sentences used to describe people's age.
She's 50 if she's a day.
Thanks

 

[5jj]

he is at least 50 - If we accept that she is at least one day old, then we must also accept that she is at least 50.

 

[emsr2d2]

Please remember to spell the word "please" in full in future (not "plz" as you put in your title). Thank you.

 

[moonlike]

Please remember to spell the word "please" in full in future (not "plz" as you put in your title). Thank you.

Sure.

 

[moonlike]

he is at least 50 - If we accept that she is at least one day old, then we must also accept that she is at least 50.

Dear 5jj
Thanks a lot.
Your explanation is really easy but really I can't get it, I can't find a relationship between being 50 and being one day? Why should we accept that she is 50 if first we accept that she is one day? By the way please correct me if I'm not right : in "she's one day", that apostrophe "s" is the short form of is not has.
P.S. Can we also say "she's 50 if she's 2 days/10 days...?

Thanks a lot and sorry for asking too many questions

 

[emsr2d2]

No, the phrase is always "... if (s)he's a day".

He says he's 25 but I swear he's 40 if he's a day.
She looks so rough. She's 60 if she's a day.

I can't give you a logical explanation for it, but it's a common enough phrase. This website shows how it can be used with different measurements, in addition to age.

etymology - Origin of "he's 6 feet tall if he's an inch" - English Language and Usage - Stack Exchange

 

[moonlike]

No, the phrase is always "... if (s)he's a day".

He says he's 25 but I swear he's 40 if he's a day.
She looks so rough. She's 60 if she's a day.

I can't give you a logical explanation for it, but it's a common enough phrase. This website shows how it can be used with different measurements, in addition to age.

etymology - Origin of "he's 6 feet tall if he's an inch" - English Language and Usage - Stack Exchange

Thanks a million dear emsr2d2
I got it, if you and 5jj didn't help,I couldn't really get it. I have never heard it. It's not common here. By the way thanks a lot for that link. What a fabulous site it is.
Thanks a lot