Musk of fear

Skaven like many rodents have glands that produce secretions for the purpose of protection. When frightened, Skaven have an involuntary response to secrete a pungent and oily liquid at the back of their neck. This oily liquid is known to Skaven as the Musk of Fear.

Secreting the Musk of Fear is to a Skaven as crapping ones pants is to a Human. It is advertising an inability to control bodily functions.