neckweed

from The Century Dictionary.

• noun A small, widely diffused plant, Veronica peregrina, once deemed efficacious in scrofula.
• noun Hemp, as used for making ropes for hangmen's use.

from the GNU version of the Collaborative International Dictionary of English.

• noun An American annual weed (veronica peregrina), with small white flowers and a roundish pod.
• noun The hemp; -- so called as furnishing ropes for hanging criminals.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An American annual weed (Veronica peregrina), with small white flowers and a roundish pod.
• noun humorous, archaic The hemp plant, which furnishes ropes for hanging criminals.