Source : Webster's Revised Unabridged Dictionary (1913)

Ultimo \Ul"ti*mo\ [L. ultimo (mense) in the last month.]
   In the month immediately preceding the present; as, on the
   1st ultimo; -- usually abbreviated to ult. Cf. {Proximo}.

Source : WordNet®

ultimo
     adj : in or of the month preceding the present one; "your letter
           received on the 29th ult" [syn: {ult}]