GNU Collaborative International Dictionary of English

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Found one definition

1.       
   -mo (?). 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; as, “sixteenmo or 16mo; eighteenmo or 18mo”.  It is taken from the Latin forms similarly used; as, “duodecimo, sextodecimo, etc.”  A small circle, placed after the number and near its top, is often used for -mo; as, 16°, 18°, etc.
   
   [1913 Webster]