Meaning of せい conjugation of する?


I came upon this line of dialogue in a book I'm reading, from a character who
has old-fashioned speech patterns:

できる限りの鶏肉を用意せい…

I assume this せい is some form of the verb する, though I'm not even sure if it's
a regional dialect or some remnant of classical Japanese. I can find plenty of
examples of the same usage on Google, but no actual explanation or grammar
notes for it in any of my usual go-to sites and reference books.

Any ideas?




Verb する has two imperative forms (命令形) with the same meaning: しろ and せよ. せい is
an old-fashioned sound variation of the latter form.




Verb する has two imperative forms (命令形) with the same meaning: しろ and せよ. せい is
an old-fashioned sound variation of the latter form.

