- -mo [mō][< ending of L abl. forms of ordinals, after prep. in, as in duodecimo (< duodecimus, twelfth)] suffix forming nounsa book, pamphlet, etc. having (a specified number of) leaves as a result of the folding of a sheet of paper a given number of times [12mo, duodecimo, or twelvemo]
English World dictionary. V. Neufeldt. 2014.