Definition of "sark"

The next thing the watchers saw was the laird struggling up the far bank and casting his coat from him, so that he rode in his sark.

1902, John Buchan, The Outgoing of the Tide

Then lorek's rear claws dug into the links of Iofur's chain-mail sark and ripped downward. The whole front came away, and Iofur lurched sideways to look at the damage, leaving lorek to scramble upright again.

2007, Philip Pullman, His Dark Materials, Bluefire, page 259