[noun] + に + あるまじき + [noun] [verb (dictionary form)] + まじき + [noun]
まじき means "that should not be": it condemns an act as improper for the person who did it. It is a literary form that always comes before a noun, and almost always as あるまじき after a role marked with に: "unbecoming of a teacher", "unworthy of a doctor".
The role before に is the standard the act falls short of, and the noun after まじき names the act: a remark, an attitude, conduct, a mistake.
まじき can follow the dictionary form of other verbs too, giving "that must not be done". Outside ゆるす許すまじき, "unforgivable", these are rare and sound archaic.
At the end of a sentence the older form まじ survives in slogans and headlines: げんばく原爆をゆるす許すまじ, "never allow the atomic bomb".
まじき belongs to news reports, editorials and formal criticism. In speech, きごう〜としてゆるされない許されない or きごう〜らしくない says the same thing.