Yesterday I talked about ～ず（に）which means “without doing ～。” 親おや= a parent, 知しら= to know, ず = NOT, so the literal meaning of 親おや知しらず is “not knowing one’s parents.”

The actual meaning of 親おや知しらず is a wisdom tooth. The reason why it’s called 親おや知しらず is that by the time children get their wisdom teeth, their parents are likely to be dead already. Now that most people live nearly 90 years, this is not the case any more but when the saying was established, probably lots of people died quite young.

There is another theory for the naming, that is, unlike most adult teeth （大人おとなの歯は/永久歯*えいきゅうし）, wisdom teeth do not replace any baby tooth （乳歯にゅうし), so they are called 親知らず (= a tooth without its own predecessor) but the first 2 molars （奥歯おくば/臼歯きゅうし*） of each side do not have their predecessors, either, so this theory is a bit weaker than the first one, I think.

Some more dental terms：
前歯まえば: a front tooth
八重歯やえば: an oblique tooth, a double tooth (In Japan 八重歯 is often considered a charm.)

虫歯むしば: a decayed tooth
入いれ歯ば/義歯*ぎし: false teeth, denture
歯医者はいしゃ/歯科医しかい *: a dentist

歯はを磨みがく: to brush one’s teeth
歯はを抜ぬく: to pull a tooth out

*Wherever 2 terms are given for the same thing, the first one is more often used in conversations and the latter is more often used in a written passage.

So, when 親知らずが痛い(= a wisdom tooth hurts), you’d better go to a dentist.