A certain species of Astragalus or Oxytropis, capable of causing locoism.
"The old loco chugged slowly up the mountain track, its steam whistle echoing through the valley."
A locomotive.
To poison with the loco plant; to affect with locoism.
"The cattle rancher inspected the pasture carefully before declaring that the grazing herd had been poisoned by the invasive loco plant."
Crazy.
"After staying up all night to finish his project, he looked completely loco when he finally arrived at work."
A direction in written or printed music to be returning to the proper pitch after having played an octave higher or lower.
"The conductor raised his hand and called out loco so that the brass section would stop playing their high octaves and drop back down to the written notes."
The word comes from Spanish loco, which originally meant "insane" or "crazy." It was also used to describe something that is loose.