pre-noun adjectival
unworthy; unbecoming; improper
A pre-noun adjectival (rentaishi) used to strongly criticize an action or behavior as not befitting someone's position, status, or character. Often follows a noun or noun phrase indicating the person or role, e.g., 教師にあるまじき行為 (conduct unbecoming a teacher).
That is a remark unbecoming of a politician.
彼の行動は指導者にあるまじきものだった。
His behavior was unworthy of a leader.
Derived from the classical Japanese negative auxiliary adjective まじ (should not) in its attributive form まじき, combined with the verb ある (to be). The expression fossilized as a rentaishi meaning 'unbecoming of' or 'unworthy of'.