Noah Webster's New International Dictionary of the English Language

1. (v. t.) To excavate the earth beneath; to subvert by indirect, secret, or underhanded means; to form a mine under; to sap; as, to undermine a wall.

2. (v. t.) Fig.: To remove the foundation or support of by clandestine means; to ruin in an underhand way; as, to undermine reputation; to undermine the constitution of the state.