Why is the ninth month called September?
1630s, earlier kag (mid-15c.), from a Scandinavian source, cf. Old Norse kaggi "keg, cask," of unknown origin. Cognate with Swedish kagge, Norwegian kagg. Specific sense of "barrel of beer" is from 1945. U.S. student slang kegger "party featuring a keg of beer" attested by 1969.