The Collaborative International Dictionary of English v.0.48:

Loco \Lo"co\, n. [Sp. loco insane.]
   1. (Bot.) A plant (Astragalus Hornii) growing in the
      Southwestern United States, which is said to poison horses
      and cattle, first making them insane. The name is also
      given vaguely to several other species of the same genus.
      Called also loco weed.
      [1913 Webster]

   2. (Bot.) Any one of various leguminous plants or weeds
      besides Astragalus, whose herbage is poisonous to
      cattle, as Spiesia Lambertii, syn. Oxytropis
      Lambertii.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Oxytropis lambertii
    n 1: tufted locoweed of southwestern United States having purple
         or pink to white flowers [syn: purple locoweed, purple
         loco, Oxytropis lambertii]