slang Utterly crazy or insane. ("Loco" means "mad" or "crazy" in Spanish, with "plumb" being used as an intensifier.) Primarily heard in US. This idea of yours is plumb loco. Poor guy went plumb loco after his wife died.
Rur. completely crazy. (Loco is from a Spanish word meaning "mad.") You're plumb loco if you think I'll go along with that. All those people were running around like they were plumb loco.
plumb loco(ˈpləm ˈloko)
mod. completely crazy. (Folksy. Loco is from a Spanish word meaning “mad.”) You’re plumb loco if you think I’ll go along with that.