Girl Archer
It turns out to be more subtle than I thought. The phrase "Atu Ankh Amen" translates as "the Living Image of Amon" (King Tut). I seem to recall that "ankh" is associated with "life" or "living," so perhaps Atu simply means "image."
The final word, though, is in the Book of Thoth itself. There is a footnote to the section titled "The Atu of Tahuti" (page 35 in my edition) that reads:
"Atu: House or Key in Ancient Egyptian."
I don't believe it is an acronym for anything, so there is no expansion that I know of.
thank you Barleywine. I just noticed your post