The Collaborative International Dictionary of English v.0.48:

-ist \-ist\ ([i^]sh"[-u]*[i^]st). [Gr. ?: cf. F. -iste.]
   A noun suffix denoting an agent, or doer, one who practices,
   a believer in; as, theorist, one who theorizes; socialist,
   one who holds to socialism; sensualist, one given to
   sensuality.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Is't \Is't\
   A contraction of is it.
   [1913 Webster]

The Free On-line Dictionary of Computing (19 January 2023):

IST

    Imperial Software Technology.

   (1995-10-12)