Source : Webster's Revised Unabridged Dictionary (1913)

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.

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}.

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

Loco \Lo"co\, n. (Bot.)
   Any one of various leguminous plants or weeds besides
   {Astragalus}, whose herbage is poisonous to cattle, as
   {Spiesia Lambertii}, syn. {Oxytropis Lambertii}.

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.

Source : WordNet®

loco
     adj : 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}, {loony},
            {loopy}, {nuts}, {nutty}, {round the bend}, {around
           the bend}, {wacky}, {whacky}]