Why is the ninth month called September?
early 15c., from be- intensive prefix + have in sense of "to have or bear (oneself) in a particular way, comport" (cf. German sich behaben, French se porter). Cognate Old English compound behabban meant "to contain," and alternatively the modern sense of behave might have evolved from behabban via a notion of "self-restraint." Related: Behaved; behaving.