Definition of "mustiness"

But during those two years for some reason or other the house had fallen into premature and rapid decay. With a very few months an air of mustiness began to hang over the once palatial residence of the rich foreign financier.

1933, Baroness Orczy [i.e., Emma Orczy], chapter I, in The Way of the Scarlet Pimpernel, New York, N.Y.: G[eorge] P[almer] Putnam’s Sons, published 1934, page 1