bittersweet nightshade

A poisonous climbing or trailing plant (Solanum dulcamara) native to Eurasia and a widespread weed in North America, having violet flowers with recurved corolla lobes and red berries. Also called bittersweet, deadly nightshade.

[After its roots, which are said to taste bitter, then sweet when chewed.]

The noun has one meaning:

Meaning #1: poisonous perennial Old World vine having violet flowers and oval coral-red berries; widespread weed in North America
  Synonyms: bittersweet, climbing nightshade, deadly nightshade, poisonous nightshade, woody nightshade, Solanum dulcamara