Crazy or insane; mentally unstable. Used informally to describe irrational or wild behavior.
From Spanish 'loco' meaning crazy or mad, which entered American English in the mid-19th century through contact with Spanish speakers in the Southwest. The Spanish word ultimately derives from Arabic 'lauqa' meaning foolish.
In the American West, 'locoweed' was a plant that made horses and cattle behave erratically when eaten, which reinforced the association between 'loco' and crazy behavior. The word perfectly illustrates how American English absorbed vocabulary from Spanish-speaking neighbors during westward expansion.
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