Extremely drunk; intoxicated to the point of being unable to function clearly (informal/slang).
British slang from early 1900s, possibly from blot (something erased or obliterated) or related to being 'blotted out' (unconscious); origin is uncertain but reflects the idea of consciousness being erased.
Blotto is a brilliant example of how slang picks up old adjectives and twists them into new meanings—calling a drunk person 'blotto' is like saying they're erased or wiped out, which is exactly how severe intoxication feels.
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