a viper in your bosom

a viper in (one's) bosom

A friend, lover, or relation who proves to be traitorous, untrustworthy, deceitful, or ungrateful. (Used especially in the phrase "nourish/nurse/nurture a viper in one's bosom.") Well, it turns out that Margaret was quite a viper in my bosom. I put my neck on the line to get her a job in our company, and then she turns around and tries to steal my position!
See also: bosom, viper

a viper in your bosom

a person you have helped but who behaves treacherously towards you.
The phrase comes from one of Aesop's fables, in which a viper reared in a person's bosom eventually bites its nurturer. The idea is also found in Latin (in sinu viperam habere ) and the expression appears in various forms in English from the late 16th century.
See also: bosom, viper