To undermine means to weaken something gradually and often secretly, like digging away the ground beneath a foundation until a structure becomes unstable. When you undermine something, you damage it from below or from within, making it less strong or effective over time.

A student who constantly interrupts the teacher undermines the class's ability to learn. Spreading rumors about a friend undermines their reputation and your friendship. When a coach's authority is undermined by players who ignore instructions, the whole team suffers because no one knows who’s really in charge.

Someone might undermine a project by quietly discouraging others from participating, or undermine their own success by procrastinating when they should be preparing.

Notice that undermining usually happens gradually rather than all at once, and often in ways that aren't immediately obvious. It's different from direct opposition or open criticism. When you undermine something, you're chipping away at its foundation rather than confronting it head-on. One late assignment might not undermine your grade, but consistently turning work in late certainly will.