Source : Webster's Revised Unabridged Dictionary (1913)

-mo \-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[deg],
   18[deg], etc.