Etymology:  <  classical Latin -mō, ablative of -mus  , suffix seen in certain ordinal numbers, as prīmus  primus adj.Originally forming the last part of graphic abbreviations (reflecting a common post-classical Latin mode of abbreviation for ordinal numerals, seen also in dates) of which the first part is a digital numeral; examples are attested in English contexts from the early 18th cent., e.g.c1716    T. Rawlinson Let. in  T. Hearne Remarks & Coll. 		(1901)	 V. 178  				I..would willingly know something of yr Sylloge Epistolar., whither MSs. & unpublish'd, or a 12mo of Ao 1640.1801    Schedule of Presswork Prices in  E. Howe London Compositor 		(1947)	 iii. 98  				Twelves: Pot, such as ladies and christian ladies table part, 6mo. 35 Pica ems wide, 26 long.1894    Amer. Dict. Printing & Bookmaking 548/1  				Trigesimo-secundo, the bibliographical term for thirty-twomo; written shortly 32mo.1927    R. B. McKerrow Introd. Bibliogr.  ii. ii. 167  				Both in a 16mo and a 32mo the watermark is, however, often absent.1949    F. Bowers Princ. Bibliogr. Descr. v. 193  				Sexagesimoquarto—64oor 64mo. Compare also the earlier use of 0:1626    S. Ward Let. in  R. Parr Life J. Usher 		(1686)	 Coll. c. 344  				A Book of a large 160. Forms in which the numerical digits are replaced by English numerals and -mo is suffixal are attested from the early 19th cent. Compare:1847    Chambers's Jrnl. 6 Feb. 87/2  				Then was let in a deluge of democratic shapes and prices. Duodecimo, post-octavo, eighteenmo, sixteenmo, and a hundred other vos and mos, bewildered the aged members of the profession.
This entry has been updated (OED Third Edition, September 2002; most recently modified version published online June 2019).