彼は無理無体に計画を進めた。
He pushed the plan forward by force.
無理無体な要求には応じられない。
I cannot comply with unreasonable demands.
強引 means 'pushy' or 'forceful' and is more colloquial. 無理無体 is more literary and implies unreasonableness beyond mere assertiveness.
A yojijukugo combining 無理 (unreasonable) and 無体 (unreasonable, forcible), both with similar meanings, to create an emphatic expression. The exact historical derivation is uncertain.