The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

proximo
    adj 1: in or of the next month after the present; "scheduled for
           the 6th prox" [syn: proximo, prox]