ultimo (adv.)youdaoicibaDictYouDict[ultimo 词源字典]
"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).[ultimo etymology, ultimo origin, 英语词源]