Exclamation. A cheer for The Simpsons character Montgomery Burns. Often used incorrectly by Canadians to suggest disproval of anything, rather than approval of Burns.
Obviously, "boo" and "Boo-urns" are two different and mutually exclusive words.
My, that old dog, Monty Burns has done it again! Boo-urns! Boo-urns, I say!