Source : Webster's Revised Unabridged Dictionary (1913)

Twenty-fourmo \Twen`ty-four"mo\, a.
   Having twenty-four leaves to a sheet; as, a twenty-fourmo
   form, book, leaf, size, etc. -- n. A book composed of sheets,
   each of which is folded into twenty-four leaves; hence,
   indicating more or less definitely a size of book whose
   sheets are so folded; -- usually written 24mo, or 24[deg].