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]