That said, for the French, it's three syllables, and English even borrows derriere for certain purposes.
But the Germanic strain in English makes one heavily predisposed toward monosyllabic words for very basic fundamental elements of existence, as one's backside certainly is.
But then, come to think on it, our favored contemporary term, "booty" makes it polysyllabic, even though we had perfectly good monosyllables available. And that has seemed to me a gain for the language.
An interesting question this: the most appropriate number of syllables in the term for one's posterior.