From An Tir Culturewiki
Just what it sounds like. Two people can agree to be each other's bedwarmers, and it doesn't matter what gender they are. Two people sharing a bed at a cold tournament are a lot warmer than one person shivering alone.
Bedwarming is strictly a platonic activity--anyone who steps over that particular boundary needs to be called on it. It never hurts to make sure that you and your prospective bedwarmer are on the same page. (Of course, if two people decide mutually to keep their bed warm by engaging in extra-curricular activities, that's strictly their business, but does go beyond the definition of the term.)
-- Mistress Arlys o Gordon