Why is the ninth month called September?
early 15c., "a joint promise to abide by an arbiter's decision," from Middle French compromis (13c.), from Latin compromissus, past participle of compromittere "to make a mutual promise" (to abide by the arbiter's decision), from com- "together" (see com-) + promittere (see promise). The main modern sense of "a coming to terms" is from extension to the settlement itself (late 15c.).
mid-15c., from compromise (n.). Related: Compromised; compromising.