Source : Webster's Revised Unabridged Dictionary (1913)

Loco \Lo"co\, n. [Sp. loco insane.] (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}.