To dig underneath (something), to make a passage or for destructive or military purposes; to sap.
(figuratively) To weaken or work against; to hinder, sabotage.
Esimerkit
Martin, for instance, had on one occasion undermined a tree sacred to old gods, then stood in the path of its fall, but forced it to fall elsewhere by making the sign of the Cross.
The growing use of social media to spread anger and dissent in the Arab world has been hailed by western governments as one of the chief justifications for a completely unfettered internet. The US is reportedly funding the secret rollout of technology in Iran in an effort to undermine internet censors in the country.
The war efforts were undermined by the constant bickering between the allies.