expression, noun
person of unknown background; suspicious stranger
Derogatory set phrase used to dismiss someone as a nobody from an unknown or untrustworthy background. Often used in the pattern どこの馬の骨かわからない.
どこの馬の骨かわからない人を家に入れてはいけない。
Don't let someone you don't know from Adam into your house.
彼はどこの馬の骨とも知れない男と結婚するなんて、正気じゃない。
She must be crazy to marry a man who's a complete nobody.
Literally 'a horse bone from somewhere', implying a worthless or unknown origin. The exact origin is uncertain, but it likely arose as a colorful derogatory expression in colloquial Japanese.