おやしらず【親知らず】 : oyashirazu | define meaning

Derived from おや【親】 oya "parents" and しらず【知らず】 shirazu "without knowing; not having". It is possible the combination arose as a popular saying alluding to the fact that wisdom teeth start growing much later in life without adults or parents even noticing them grow in. Alternatively, it refers to the fact that wisdom teeth do not have a corresponding set of baby teeth.