親知らず

Short for 親不知歯 (oyashirazu-ba, literally “parents not-knowing tooth”). a wisdom tooth (from the likelihood that the wisdom teeth do not erupt until after no longer in daily contact with one's parents, due to moving or death)

ignorance of one's parentage; such a person who does not know their parents

(historical) a specific kind of formal adoption during the Edo period, wherein the adoptee would have no knowledge of their birth parents; such an adoptee

(regional, possibly archaic) a section of coast with dangerous surf (by analogy from having to pay all attention to surviving the conditions, and not having a moment to spare to think of one's parents)