Why is the ninth month called September?
early 14c., from Old French (arc) botrez "flying buttress," apparently from bouter "to thrust against," of Frankish origin (cf. Old Norse bauta "to strike, beat"), from Proto-Germanic *butan, from PIE root *bhau- "to strike" (see butt (v.)).
late 14c., literal and figurative, from buttress (n.). Related: Buttressed; buttressing.