any of several venomous Old World snakes of the genus Vipera, esp. V. berus, a small snake common in northern Eurasia.

any related snakes belonging to the family Viperidae, characterized by erectile, venom-conducting fangs.

pit viper.

any of various venomous or supposedly venomous snakes.

a malignant or spiteful person.

a false or treacherous person.

( initial capital letter ) Military . a 9-pound (4 kg), shoulder-launched, unguided U.S. Army antitank rocket with an effective range of 273 yds. (250 m). ?

to nourish a viper in one's bosom

to befriend a person who proves to be treacherous.