I think it dates back to the time when a decent shirt was an expensive
luxury item that the 'shirty proponent' would prefer not to get
bloodstained by his opponent's blood spatters (fisticuffs or sword
wounds). Also, of course, it would protect against sweat stains due to
his exertions which would indicate to his opponent that he was prepared
to get 'stuck in' and not back off[1]. And, finally, any flesh wounds
(gunshot or knife) would be less likely to become seriously infected by
cloth fragments forced into the deeper flesh by such weaponry.

[1] In this context, "Getting Shirty", the most common interpretation.


Charles I must have been rich then:-)

He could afford two shirts.

--
Adam