What does loco mean?
“Loco” is chiefly an informal word for someone or something crazy, wildly eccentric, or extremely excited, as in “The fans went loco when their favourite entered the stage.” It can function as an adjective, adverb, or verb, so both “an absolutely loco idea” and “the delay drove me loco” are natural. Its older, still-established noun sense refers to a locomotive or railway engine, especially in technical contexts and Indian English; a further uncommon sense denotes locoweed, a plant poisonous to livestock. The word conveys exaggeration and playful intensity more often than a precise medical condition, making it particularly effective in informal speech. Although “loco” may sound archaic because of its association with old railways, it remains common in North American, British, and Indian English. Its emphatic, memorable character makes it useful whenever ordinary language seems too restrained to express astonishment, frustration, or exuberance.
Informally, crazy, eccentric, or behaving in a wildly excited or uncontrolled manner.
"The entire audience became absolutely loco after the final performance."
Informal and often hyperbolic rather than medical.
A locomotive or railway engine.
"The freight locomotive arrived shortly before dawn."
The technical sense is standard; Indian English also uses the word commonly for a railway engine.
Any of several poisonous plants of the pea family that can cause livestock to behave erratically or become ill.
"Ranchers watch carefully for locoweed in grazing areas."
Chiefly associated with livestock and regions where the plants occur.
Informally, in a crazy, eccentric, or wildly excited manner.
"The band played loco throughout the encore."
Informal and strongly emphatic.
The usual plural of the noun meaning a locomotive, especially in technical or Indian English. The informal adjective is normally used predicatively and does not require a plural form.
"Two old locos were parked beside the maintenance shed."
Before it became a punchy synonym for crazy, “loco” was also short for locomotive—a fitting double life for a word that can send a train off the tracks or send its listener off their mind.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does loco come from?
The familiar modern sense of “loco” was borrowed from American Spanish, where “loco” means “crazy” or “insane.” English usage developed from the late nineteenth century, while the separate railway sense is an abbreviation of “locomotive.” The term ultimately belongs to a family of words connected with motion, including “locomotion” and “locomotive.”
Related word forms
How loco is actually used
The adjective, adverb, and verb are informal and often humorous rather than seriously diagnostic; the noun meaning a railway engine is formal or technical, while the Indian English use is established but geographically distinctive. The locoweed sense is relatively uncommon and regional in practical relevance.
Easily confused with loco
“Loco” in its informal modern use means crazy or wildly excited, whereas “local” means relating to a particular place or area.
“Loco” is pronounced in two syllables and may mean crazy or a railway engine, while “lock” is pronounced with one syllable and can mean a fastening device.