'Compromise' meant 'mutual promise to accept an arbiter' — settling by concession came later.
A settlement of differences in which each side makes concessions; to settle a dispute by mutual concession; to weaken or damage by accepting standards that are lower than is desirable.
From Old French 'compromis' (a mutual promise to abide by an arbiter's decision), from Latin 'comprōmissum' (a mutual promise to refer a dispute to arbitration), neuter past participle of 'comprōmittere' (to make a mutual promise), composed of 'com-' (together, mutually, with) + 'prōmittere' (to promise, to send forward, to let go), from 'pro-' (forward) + 'mittere' (to send, to let go). The PIE root behind 'mittere' is *meyth₂- or *mit- (to send, to throw). The original sense was not settling
In most Romance languages, the cognate of 'compromise' does not mean settling by concession — it means a binding commitment or engagement. Spanish 'compromiso' means an obligation or engagement; Italian 'compromesso' can mean a deal. The English sense of 'settling by mutual concession' developed only in the seventeenth century, while the negative sense of 'compromising' one's principles or security came even later