The Collaborative International Dictionary of English v.0.48:

Loco \Lo"co\, adv. [It.] (Mus.)
   A direction in written or printed music to return to the
   proper pitch after having played an octave higher.
   [1913 Webster]

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.]

The Collaborative International Dictionary of English v.0.48:

Loco \Lo"co\, v. t. [imp. & p. p. Locoed; p. pr. & vb. n.
   Locoing.]
   To poison with loco; to affect with the loco disease; hence
   (Colloq.), to render insane or mad. "The locoed novelist."
   --W. D. Howells.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Loco \Lo"co\, n.
   A locomotive. [Colloq.] --Kipling.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

loco \lo"co\, a. [Sp. loco insane.]
   Insane; crazy. [Originally Southwestern U. S., now slang]
   [PJC]

WordNet (r) 3.0 (2006):

loco
    adj 1: informal or slang terms for mentally irregular; "it used
           to drive my husband balmy" [syn: balmy, barmy,
           bats, batty, bonkers, buggy, cracked,
           crackers, daft, dotty, fruity, haywire,
           kooky, kookie, loco, loony, loopy, nuts,
           nutty, round the bend, around the bend, wacky,
           whacky]