overstepping one's authority; abuse of power; acting ultra vires
Formal legal or bureaucratic term for exceeding one's official powers or competence. Rare in casual conversation.
The official's actions were criticized as an overstepping of authority.
越権 is a more common term for exceeding authority, without the nuance of 'presumptuousness' implied by 僭上.
僭越 is a humble expression used when one feels they are overstepping their bounds, often in formal speech.
A yojijukugo (four-character compound) combining 僭上 (presumptuousness, overstepping rank) and 越権 (exceeding authority). The exact historical derivation is uncertain.