Cowbow talk for rope; in verb form to hang (someone). Hemp fever was a morbidly jocular term for a hanging. Hemp party (also string party) meant the same. A hemp committee was a group of vigilantes or a lynch mob (depending on your point of view) and a hemp necktie was the rope they did the deed with.

Cool Western Slang
http://www.bibble.org/western_slang.html