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!

Also, snake in one's bosom. An ungrateful or treacherous friend, as in I got him dozens of freelance jobs, and then he told everyone I was a lousy musician-nothing like nourishing a viper in one's bosom . This metaphoric expression, often put as nourish a viper (or snake) in one's bosom, comes from Aesop's fable about a farmer who shelters a snake dying from the cold, which then fatally bites him after it recovers. It was referred to by Chaucer and Shakespeare, and appeared in numerous proverb collections.

An ungrateful and treacherous friend; an individual whom one has helped and who returns the favor with treachery. This term comes from Aesop’s fable about a peasant who brings indoors a snake dying from the cold and is fatally bitten as soon as the snake recovers. Chaucer was among the first of the many writers to use this metaphor, which also made its way into the proverb collections of John Ray (1670), James Kelly (1721), and Thomas Fuller (1732). The term is variously put as snake or viper.