E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The private rooms of a house; the secrets of a family. That part of a Roman temple into which the priest alone had access; here were the sacred images, here the responses of the oracles were made, and here the sacred mysteries were performed. The Holy of Holies was the penetralia of the Jewish Temple. (Latin plural of penetrlis.)