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).