What it means
まじき is the attributive form of the classical auxiliary まじ, attached to a verb in the dictionary form. It expresses a strong judgment that an action must not be performed, should not be allowed, or is unacceptable in relation to the following noun. The construction is especially common in formal writing, official statements, criticism, and fixed expressions describing conduct or states that violate a standard.
Usage note
まじき is highly literary and formal, so it is not a natural choice for ordinary conversation. It usually appears before a noun, and the following noun is often evaluated as unacceptable, such as an act, mistake, behavior, or policy. It can be followed by either だ or です, but changing the copula does not make the construction casual.
How to use it
それは二度と繰り返すまじき過ちです。
That is a mistake that must never be repeated.
それは二度と繰り返すまじき過ちだ。
That is a mistake that must never be repeated.
Example sentences
市民の安全を軽視するまじき政策です。
It is a policy that must not disregard the safety of citizens.
患者の命を預かる医師にあるまじき判断です。
It is a judgment unacceptable for a doctor entrusted with patients' lives.
高齢者を狙った詐欺の増加は、社会全体が看過