1. to go against somebody, to work against, to make someone look bad
John completely undermined me in that meeting, undermine sb's authority
The verb 'to undermine' means to reduce the effect, power or ability of someone or something, usually incrementally or over a period of time. When you 'undermine' someone you might show that they are wrong or make them lose face in some way. It can also mean 'to erode' if you are talking about a rock or the base of something.