early 15c., "a joint promise to abide by an arbiter's decision," from Old French compromis (13c.), from Late Latin compromissus, past participle of compromittere "to make a mutual promise" (to abide by the arbiter's decision), from com "with, together" (see com-) + promittere "to send forth; let go; foretell; assure beforehand, promise," from pro "before" (from PIE root *per- (1) "forward," hence "in front of, before") + mittere "to release, let go; send, throw" (see mission).
The sense of "a coming to terms, a settlement of differences by mutual concessions" (mid-15c.) is from extension to the settlement itself. The meaning "that which results from such an agreement" is from 1510s.