ultimo

[uhl-tuh-moh] /ˈʌl təˌmoʊ/
adverb
1.
in or of the month preceding the current one:
on the 12th ultimo.
Abbreviation: ult., ulto.
Compare instant (def 11), proximo.
Origin
1575-85; < Latin ultimō (mēnse or diē) in the last (month) or on the last (day)
British Dictionary definitions for ultimo

ultimo

/ˈʌltɪˌməʊ/
adverb
1.
(esp abbreviated in formal correspondence) in or during the previous month: a letter of the 7th ultimo, ult. Compare instant (sense 7), proximo
Word Origin
C16: from Latin ultimō on the last
Word Origin and History for ultimo

"in the month preceding the present," 1610s, common in abbreviated form ult. in 18c.-19c. correspondence and newspapers, from Latin ultimo (mense) "of last (month)," ablative singular masc. of ultimus "last" (see ultimate). Earlier it was used in the sense of "on the last day of the month specified" (1580s). Contrasted with proximo "in the next (month)," from Latin proximo (mense).