From the second edition (1989):
‖ultimo, a. and adv.
(ˈʌltɪməʊ) [L. ultimō (sc. diē or mense), abl. sing. masc. of ultimus last. So in G., Du., Sw., etc.]
†1. On the last day (of a specified month). Obs.
1582 Allen Martyrd. Fr. E. Campion (1908) 17 In the xxij yere of the raigne of our soveraine Lady the Queene, Maij vltimo. 1682 J. Scarlett Exchanges 102 If it be dated ultimo February, then its not due till the ultimo March. Ibid., If for the 30th of June he write ultimo, it will not be due till ultimo July.
2. Of last month. (Abbreviated ult. and ulto.)
1616 R. Cocks Diary (Hakl. Soc.) I. 125, I receved a letter from Mr. Wickham, dated‥the 22th ultimo. 1683 W. Hedges Diary (Hakl. Soc.) I. 63 Letters from Cassumbazar advised Thomas Bromly dyed ye 29 Ultimo. 1754 Washington Let. Writ. 1889 I. 70 The 25th ultimo,‥I received ye news of your Honour's arrival. 1792 Ibid. (1891) XII. 242, I was very glad to receive your letter of the 31st ultimo. 1823 P. Hawker Diary (1893) I. 261 The morning of the 31st ultimo. 1841 Hawthorne in J. Hawthorne N. Hawthorne & Wife (1885) I. 227, I took up my abode here on the 12th ultimo.