Past tense of barf: to vomit or throw up.
American slang from the 1950s, likely onomatopoeia imitating the sound of vomiting. The origin is uncertain but may relate to 'blarf' or similar sound-words.
The word 'barf' is purely American slang that became so common it now appears in dictionaries as standard English—British people would never say it, they'd say 'was sick' or 'vomited'!
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