adverb / noun
ultimo
UL-tuh-moh/ˈʌltəˌmoʊ/
Referring to the previous month in dated correspondence.
I acknowledge receipt of your letter of the 12th ultimo.

Etymology
Latin ultimus meant 'last'; in the expression behind ultimo, the understood noun was 'month.' Thus 'last' identifies the preceding calendar month rather than an unspecified recent time.
More examples
- Your invoice of the 5th ultimo remains unpaid.
- We refer to the agreement signed on the 20th ultimo.
- In a letter dated June 8, “3rd ultimo” means May 3.
Forms
ultimo