from The American Heritage® Dictionary of the English Language, 4th Edition
- suffix Used after numerals to indicate the number of leaves that results from folding a sheet of paper: twelvemo.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- suffix Used to form page and book sizes.
from the GNU version of the Collaborative International Dictionary of English
- A suffix added to the names of certain numerals or to the numerals themselves, to indicate the number of leaves made by folding a sheet of paper. It is taken from the Latin forms similarly used; A small circle, placed after the number and near its top, is often used for -mo; as, 16°, 18°, etc.
From (duodeci)mo.(American Heritage® Dictionary of the English Language, Fourth Edition)
From Latin ending and apparent suffix of cardinal numbers. (Wiktionary)