1.to be incumbent on sb or to be incumbent upon sb<正式>是某人义不容辞的责任
It is incumbent upon all of us as loyal citizens to make an extra effort.作为忠诚的公民, 我们每个人都有义务加倍努力。
2.+ politician, official在职的;现任的
the only candidate who defeated an incumbent senator唯一击败现任参议员的候选人