nourish a viper in one's bosom

From Aesop's fable The Farmer and the Viper.

nourish a viper in one's bosom (third-person singular simple present nourishes a viper in one's bosom, present participle nourishing a viper in one's bosom, simple past and past participle nourished a viper in one's bosom)

1. (idiomatic) To do acts of kindness that will be met with ingratitude and will cause one harm in the end.

   Synonym: cherish a snake in one's bosom