/ ˌʌndəˈmaɪn; NAmE -dərˈm-/ verb [ vn ]
1.
to make sth, especially sb's confidence or authority, gradually weaker or less effective :
Our confidence in the team has been seriously undermined by their recent defeats.
This crisis has undermined his position.
2.
to make sth weaker at the base, for example by digging under it
••
WORD ORIGIN
Middle English : from under- + the verb mine , probably suggested by Middle Dutch ondermineren .