Why is the ninth month called September?
"agree, fit," 1813, of unknown origin, perhaps a figurative extension of earlier jib, gybe (v.) "shift a sail or boom" (see jib). OED, however, suggests a phonetic variant of chime, as if meaning "to chime in with, to be in harmony." Related: Jibed; jibes; jibing.
1560s, perhaps from Middle French giber "to handle roughly," or an alteration of gaber "to mock."