The Collaborative International Dictionary of English v.0.48:

inst \inst.\ adj. [abbreviation for instant.]
   in or of the present month; same as instant[3], a. or
   instant[2], n.; as, your letter of the 10th inst.. [abbr.]

   Syn: instant.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

inst
    adj 1: in or of the present month; "your letter of the 10th
           inst" [syn: instant, inst]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

INST
       INformation Standards and technology Standardization