A practitioner or supporter of an ism
"The new ist joined the group to advocate for their beliefs."
"The new shop owner is looking for an experienced assistant to help with sales on weekends."
Initialism of incompetent to stand trial.
"The ist quarter is often used in historical texts to refer to January."
Abbreviation of first.
In plain English: The suffix -ist simply means someone who is an expert at, practices, or strongly believes in something specific.
"The book is very interesting to read."
Initialism of InterSwitch Trunk.
"The network administrator documented that IST represents an InterSwitch Trunk in the server configuration manual."
The word ist comes from a common English suffix that originally meant one who practices or is an expert in something. It entered modern usage to form nouns like artist and scientist by attaching directly to other words.