Definition of dun:
(n) :
a brownish grey colour; a collector of debts
"Melancholy duns came looking for him at all hours", G. Orwell, Down and Out in Paris and London, Ch. 18
(adj) :
of a brownish grey colour
(v) :
to ask or beset a debtor for payment
"no wonder she is in a bad mood, since she did nothing but dun customers all day."