Definitions of betray

to not be loyal to something or someone to which you should be loyal, especially to a country or a person eg by giving away secrets or by helping their enemies

Did her sister betray her to the enemy?

to not do as you promised

The Government has once again betrayed its election promises.

to reveal something about yourself that you have tried to hide or would not want revealed

This is no liberal conspiracy; the president's own words betray his prejudices.