The expression “a hurrah’s nest” means: a disorderly, untidy mess; a place of wild confusion.
This “nest” has been variously attributed to a hurrah, a hurra, and a hoorah, but, I regret to say, no naturalist or folklorist has ever yet attempted to describe the imaginary creature responsible for the untidiness.
My one-time associates, compilers of The Standard Dictionary of Folklore (1949-50), ignore it.
In fact, because it first appeared in Samuel Longfellow’s biography of his brother, Henry Wadsworth Longfellow, in 1829, and next in Twenty Years Before the Mast (1840), by Richard Henry Dana, who was a student at Harvard under H. W. Longfellow, it may well be that the “hurra’s nest,” as the biographer wrote it, was a family term that entered unconsciously into the ordinary conversation of members of the Longfellow family and was picked up by others who heard them use it.
Our language does sometimes grow in that manner.