E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A Sanscrit word, somewhat similar to Amen. When the gods are asked to rejoice in a sacrifice, the god Savitri cries out Om (Be it so). When Pravâhan is asked if his father has instructed him, he answers Om (Verily). Brahmans begin and end their lessons on the Veda with the word Om, for unless Om precedes his lecture, it will be like water on a rock, which cannot be gathered up; and unless it concludes the lecture, it will bring forth no fruit.
Om mani padëm hûm. These are the first six syllables taught the children of Tibet and Mongolia, and the last words uttered by the dying in those lands. It is met with everywhere as a charm.