"Keep your shirt on"

[Paul McFedries]

I'm trying to track down the origin of the phrase "keep your shirt on,"
which means "don't get angry or upset." So far, I have a couple of sources
that trace it back to the ancient Viking warriors called "berserkers."
Apparently, they were quick to anger and, in their fury, would rip off their
shirts and fight bare-chested. Assuming this is a reasonable explanation, I
was wondering if there is any connection to the word berserker, which
apparently means "bear shirt" in Old Norse.

Thanks a kiloton.

Paul