take（it）（up）on oneself
assume powers beyond what one should超出个人权力范围承担（某事）；擅自
He took it upon himself to cancel the contract without consulting any of his colleagues．他不同任何同事商量就擅自取消了合同。
That fellow takes too much upon himself；anyone would think he was the manager．那个家伙太会自作主张多管事，任何人都会以为他是经理。