Why is the ninth month called September?
early 13c., "to depart, make off;" early 14c., "to turn aside, deviate from a straight course," probably from Old English sweorfan "to rub, scour, file" (but sense development is difficult to trace), from Proto-Germanic *swerbanan (cf Old Norse sverfa "to scour, file," Old Saxon swebran "to wipe off"), from PIE root *swerbh-. Cognate words in other Germanic languages (cf. Old Frisian swerva "to creep," Middle Dutch swerven "to rove, stray") suggests the sense of "go off, turn aside" may have existed in Old English, though unrecorded. Related: Swerved; swerving.
1741, from swerve (v.).