A bad mistake or failure, or when something goes badly wrong; a mess-up or bungled situation.
British slang from 'cock' (a rooster, or colloquial for penis) + 'up' (upward), possibly meaning 'to make stand up wrong' or from the sense of 'cocking up' a gun. The term became widespread in 20th-century British English.
This is 100% British slang that Americans would call a 'screw-up,' and it's a great example of how the same language describes mistakes differently depending on where you live!
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