[noun][from at least 1595]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)
[noun][date uncertain, likely late 1100s]ignorance of one's parentage; such a person who does not know their parents
[noun][date uncertain, some time after 1100s](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)
[proper noun][roughly late 1100s] A cliff-lined coast in Itoigawa, Niigata Prefecture, Japan
[proper noun][date uncertain, some time after late 1100s] A cliff-lined coast in Shizuoka, Shizuoka Prefecture, Japan
(This term, 親不知, is an alternative spelling of the above term.)