ultimo

from WordNet (r) 3.0 (2006)

ultimo
    adj 1: in or of the month preceding the present one; "your
           letter received on the 29th ult" [syn: {ultimo}, {ult}]

from The Collaborative International Dictionary of English v.0.48

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}.
   [1913 Webster]