Merriam-Webster Collegiate Dictionary - loco

 I. adverb or adjective  Etymology: Italian dialect, there, from Latin in ~ in the place  Date: circa 1801 in the register as written — used as a direction in music  II. noun  (plural ~s or ~es)  Etymology: Mexican Spanish, from Spanish, crazy  Date: 1844  1. ~weed  2. ~ism  III. transitive verb  Date: 1884  1. to poison with ~weed  2. to make frenzied or crazy  IV. adjective  Etymology: Spanish  Date: 1887 slang mentally disordered ; crazy, frenzied