Don't bother arguing with someone who refuses all reason.
Tips
usage
Strongly negative - used of people, not situations. The person is aggressively irrational: won't listen, ignores facts, blusters. Different from 不讲道理 (just unreasonable) - 蛮 adds a rough, bullying tone.
memory
蛮 originally meant 'uncivilized southern tribes' in classical Chinese, now 'rude/brutish'. 蛮 + 不讲理 = 'brutishly refusing to be reasoned with'.
