〜まじき
unacceptable for (someone in a role)
Role + に / として + あるまじき + Noun (行為, 発言, 態度)
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まじき is a classical "should not", used almost only in the set phrase あるまじき, "unbecoming of": 教師にあるまじき行為だ, "that's behaviour unbecoming of a teacher".
The pattern is Role + に / として + あるまじき + Noun. The role is a respected position (teacher, politician, doctor, police officer), and the noun is behaviour: 行為, 発言, 態度 or ミス. The tone is strong condemnation, the kind you hear in news reports of scandals.
Occasionally, it follows other verbs: 許すまじき, "unforgivable".
It's the adjectival form of まじ, the negative of べし, so it's a partner of べからざる. Compare ともあろう, which criticises someone for acting below their status: 教師ともあろう者が….
Examples
それは教師にあるまじき行為だ。
That's behaviour unbecoming of a teacher.
政治家としてあるまじき発言だ。
That's a remark no politician should ever make.
医者にあるまじきミスだった。
It was a mistake a doctor should never make.
それは人として許すまじき行為だ。
As a human being, that's an unforgivable act.
警察官にあるまじき態度だ。
That attitude is unbecoming of a police officer.