The Collaborative International Dictionary of English v.0.48:

Contrayerva \Con`tra*yer"va\, n. [Sp. contrayerba, literally, a
   counter herb, hence, an antidote for poison, fr. l. contra +
   herba herb.] (Bot.)
   A species of Dorstenia (Dorstenia Contrayerva), a South
   American plant, the aromatic root of which is sometimes used
   in medicine as a gentle stimulant and tonic.
   [1913 Webster]