Why is the ninth month called September?
early 14c., "to turn upside down, to overthrow," from over- + Middle English whelmen "to turn upside down" (see whelm). Meaning "to submerge completely" is mid-15c. Perhaps the connecting notion is a boat, etc., washed over, and overset, by a big wave. Figurative sense of "to bring to ruin" is attested from 1520s. Related: Overwhelmed; overwhelming; overwhelmingly.