I did not know that! Always thought it was something to do with Trojan horses and that you're assuming the gift is a trap, which would be an impolite thing to assume.
TIL that the phrase "don't look a gift horse in the mouth" exists because horses get more teeth as they age, so examining a gift horse's mouth is rude, as you're assuming they are of questionable age and quality. 
https://www.bloomsbury-international.com/student-ezone/idiom-of-the-week/1797-dont-look-a-gift-horse-in-the-mouth/ …
-
-
-
Your idea makes more sense (as usual)
Kraj razgovora
Novi razgovor -
Čini se da učitavanje traje već neko vrijeme.
Twitter je možda preopterećen ili ima kratkotrajnih poteškoća u radu. Pokušajte ponovno ili potražite dodatne informacije u odjeljku Status Twittera.