This article is from the Pratchett Newsgroups FAQ, by Orin Thomas firstname.lastname@example.org with numerous contributions by others.
Merkin is an affectionate name used by some of the denizens of
alt.fan.pratchett for citizens of the United States. If you want
to know what the word really means I suggest that you consult a good
Apparently on one of the larger US networks AFP's newsgroup description
says "Book signings and the sale of Merkins". One unfortunate
actually posted asking where could he order a merkin. Apparently he needed
one due to an embarrasing personal problem.
No one on AFP sells Merkins. (although it could signal a strange new
diversification of the CMOT Briggs enterprise :-). CMOT replied that
he didn't really want to explain all this to the British Postal Service.