[noun](historical) The inner chamber (inner quarters) of the Edo Castle where the shōgun's consort, concubines, and ladies in waiting resided. Other than the shōgun, no other men were permitted entrance. Male children were raised in Ooku until the age of 7 at which time they had to move out and reside in other parts of the castle.
(This term, おおおく, is the hiragana spelling of the above term.)