Loco means mad, in informal use. Go loco, completely loco, drive him loco.
It is American slang taken from Spanish and it is light-hearted, which is why it is used where a serious word about mental illness would be wrong. Loco is also a short form of locomotive, which is a different word entirely, from Latin.
1adjectivelocoMad, in informal use.
“You're going out in this storm? Have you gone completely loco?”
loco is a state.
Also accepted in games: bonkers
Spanish loco means insane. It entered American English through the south-west in the nineteenth century.
Borrowed from Italian -.
Etymology from Wiktionary, CC BY-SA