I first heard this phrase in an episode of Family Guy, and they're typically fans of referencing older shows and movies, especially from the 80s. So I'd assumed it was a fairly commonly known thing.
Then I came across it in a friend's novel. The novel is in a Southern American, older time setting, which is about where I'd expect to find examples of it.
Then I had a look online and couldn't find examples of the phrase or it's origin.
Is there such a phrase and origin? Or is this just a coincidence with people choosing something that sounds equally old-timey?
Best Answer
An expression whose logic I never understood. A typical example is
Meaning he is at least fifty. The addition "if he is a day" is a hint that it is a guess but one is rather sure that the estimation is not wrong.
At the moment my hypothesis is "if he is a day older or so it does not matter". But I am not very sure and I would prefer some evidence for it, which would necessitate longer researches in older literature, dictionaries or corpuses. But corpuses don't go back to older language variants. Perhaps someone else has or can find some evidence.
There is a long thread on the forum of wordreference.com. The general consensus is it is a guess, meaning "at least/or more". Some views are it means "if he is a day old", but that I think is drawn out of the finger.
http://forum.wordreference.com/threads/if-he-is-a-day.79171/
Added: A year ago or so I noted down this formula as unintelligible. Though I have been trying to find a logical derivation I haven't found one. But now an idea comes to my mind: "He is fifty if he is a day" means He is fifty, that's my guess and I'm rather sure. If he is a day less than fifty that doesn't make my guess wrong. - Well, something of this kind. I suppose the part with "if he is a day" is a hint that the whole formula consisted of two or even three sentences. But who knows what these sentences were. Etymonline has an entry for day but doesn't even hint at this formula, let alone explain it.