loco

American  

[loh-koh] / ˈloʊ koʊ /

noun

plural

locos

1. locoweed.

2. Slang. an insane person; maniac.

3. Veterinary Pathology. locoism.

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verb (used with object)

locoed, locoing

1. to poison with locoweed.

2. Slang. to cause to be insane or crazy.

adjective

1. Slang. out of one's mind; insane; crazy.

loco ¹ British  

/ ˈləʊkəʊ /

adjective

1. slang insane

2. (of an animal) affected with loco disease

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noun

1. short for locoweed

verb

1. to poison with locoweed

2. slang to make insane

loco ² British  

/ ˈləʊkəʊ /

noun

1. informal short for locomotive

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loco ³ British  

/ ˈləʊkəʊ /

adjective

1. denoting a price for goods, esp goods to be exported, that are in a place specified or known, the buyer being responsible for all transport charges from that place

   loco Bristol

   a loco price

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Etymology

Origin of loco

1835–45, < Spanish: insane

Example Sentences

Coaches often serve in loco parentis, guiding a developing prospect not only through training and tournaments but also the awkward riddles of growing up.

From The Wall Street Journal • Dec. 23, 2025

"The loco pilot, on observing the herd of elephants, applied emergency brakes. However, elephants dashed with the train," he said.

From Barron's • Dec. 20, 2025

On the return journey, a bolt sheared on what is now more widely known as the Penydarren loco, causing the boiler to leak.

From BBC • Feb. 24, 2024

Hugon: Nunca me dio vergüenza, pero a veces la gente no entiende qué es la terapia y piensa que uno está loco.

From Los Angeles Times • Aug. 16, 2023

And although admiring Joe Mondragon, they also thought he was a jerk—you had to be loco to ask for a kick in the ass like that.

From "The Milagro Beanfield War" by John Nichols