What is the meaning of 「ちょいとまった」?

Question

I think it is came from 「ちょっとまって」and would be alternated by a certain dialect and archaic usage.

I would like to know why the manga character said it in the past tense.

Answer 1

It could be said semi-archaic variant of ちょっと. I've never heard people around me use it unless they want to speak in a jocular way. It's also (stereo-)typically associated with over-middle-age people, older school of delinquent youth or yakuza, or roughly Edo to pre-war period accents in many fictional works.

As for dialects, online dictionaries on Gunma, Yamanashi, and Wakayama dialects seem to have this word as headword.

Don't confuse it with nominal suffix -ちょい, which is still in regular use as a more casual variation of nominal suffix -ちょっと "a little over; -odd".

For the "past tense" part, see @chocolate's comment.