Subpoena comes from the first two words of a legal writ which required a person to comply with some order of the court; it means "under penalty".

Under penalty, get your butt before me so I can try you, for example.

Writs were very long, often hundreds of words, and this is just a shorthand description of one of them.

Habeas corpus is another one.

And then there's subpoena duces tecum. That's a writ requiring the production of some item by the person upon whom the writ is served, usually a document, to the court. It means literally Under penalty, carry with you . . . and there followed more Latin along with a description of the items to be carried into court.

If you get a copy of Black's or a similar Law Dictionary and leaf through it you will find hundreds of these ancient writs, all described by the first two, three, perhaps four words of the original Latin in which they were written.