"don't be such a Jeremiah"

Jeremiah lamenting the destruction of Jerusalem
Public DomainJeremiah lamenting the destruction of Jerusalem - Credit: Rembrandt van Rijn
Jeremiah was a Biblical prophet whose writings form the Book of Jeremiah and possibly the Book of Lamentations.  In The Far Pavilions, when Wally tells Ash not to be "such a Jeremiah", he is referring to Ash's pessimistic predictions of disaster.