〜まじき

Meaning

Indicates something that must not be, given one's position or reason. An archaic literary expression modifying a noun with strong condemnation, as in 「許すまじき」 (unforgivable) or 「あるまじき」 (inexcusable, unbecoming).

Examples

それは教育者にあるまじき行為だ。 That is conduct unbecoming of an educator.
弱い者をいじめるなど、許すまじき卑劣なふるまいだ。 Bullying the weak is an unforgivable and despicable act.