The beam in my own eye

I like Nora Roberts and have read many of her books, including some that feature sorcery and celtic mythology. One thing that I objected to in those however was her use of the word "mote" in spells - eg

And each cross of silver a shield will be. As we will, so mote it be.

[from Morrigan's Cross]

The silliness of rhyming spells was bad enough; but "mote", I thought, was not an auxilliary verb, and I couldn't find it in the Concise Oxford either.

You have to be careful, though, before you castigate a favourite, so I looked for "mote" on the web and lo!

http://www.yourdictionary.com/mote

I see now that it's also in my 1955 edition of the Shorter Oxford Dictionary. ("Shorter", by the way, does not mean short: the SOD contains over two and a half thousand large format pages and weighs eight pounds.)