adverb
by fair means or foul; right or wrong; no matter what
Fixed expression meaning 'whether it's right or wrong', used to insist on doing something regardless of the consequences or morality. Often implies a stubborn determination.
理が非でも、彼は自分の意見を押し通そうとした。
Right or wrong, he tried to push through his own opinion.
理が非でも、この計画
By fair means or foul, I intend to put this plan into action.
Compound of 理 (り, 'reason, logic') + が (subject marker) + 非 (ひ, 'wrong, injustice') + でも ('even if'). Literally 'even if reason is wrong', it became a set phrase for doing something regardless of right or wrong.