I think "arder" has no really meaning here.
I always though it refers to the medieval ages, when havin a good big beard was a sign of autority and adulthood. When someone was powerfull enough to cut other's beard, it was good to soak your own and make the thign more easy (and less painful
)
Here there is a very good reference (but with other meaning)
reference here:
http://www.finlay-online.com/albarra...olconozca6.htm
So the usual version it's the one aleccowan has given, and the other a misunderstanding --> You must soak your beard in order not to be burn.
Saludos