Here's why:
* Interjections are words or phrases that express strong emotions or sudden feelings.
* "Bah Humbug!" is a classic exclamation of disdain or disapproval, particularly associated with the character Scrooge in Charles Dickens' *A Christmas Carol*.
While "Bah" and "Humbug" are technically nouns in other contexts, they function as a single unit to express a feeling in this specific phrase.