Source : Webster's Revised Unabridged Dictionary (1913)

Proximo \Prox"i*mo\ [L., on the next, abl. of proximus next.]
   In the next month after the present; -- often contracted to
   prox.; as, on the 3d proximo.

Source : WordNet®

proximo
     adj : in or of the next month after the present; "scheduled for
           the 6th prox" [syn: {prox}]