Imagine the skeleton of a horse half-buried in the sand. Who knows who the horse was? What of the horse’s master? Nobody knows. That’s the feeling behind the expression when applied to a newcomer in the tight-knit Japanese society. Uma no hone carries a heavy negative connotation when referring to an outsider.
A: Kekkon mae no musume ga doko no uma no hone da ka wakaranai yoona otoko to tsukiatte wa ikemasen yo.
A: Before marriage a girl should not go out with a man of unknown background.