Green’s Dictionary of Slang

(orig. milit.) to blunder, to make a mess of; often as cock it up.

1959		G. Slatter Gun in My Hand 162: I cocked up my exams.
1965		J.P. Carstairs Concrete Kimono 30: O.K. I’d goofed (for U.S. readers) or cocked-up (for the British).
1973		B.S. Johnson All Bull 60: They cocked things up somehow in my case.
1977		(con. 1960s) Nicholson & Smith Spend, Spend, Spend (1978) 185: I cocked it up.
1989		M. Amis London Fields 418: It sounds as though you’ve cocked things up nicely.
1993		K. Lette Foetal Attraction (1994) 181: I like the olden days [...] If a doctor cocked up, his hand got amputated.
1999		Guardian G2 13 July 3: Labour have completely cocked it up.
1999		K. Sampson Powder 232: Tell the band that the promoter’s cocked it up.
2000		Guardian 4 Mar. 22: As though Doreen Lawrence and Sukhdev Reel would be happier if the investigations of their son’s deaths had been cocked-up by officers who weren’t white.
2001		M. Coles Bible in Cockney 62: He tells the demons where to go, and he forgives people who cock things up – who sin.
2003		Bug (Aus.) July 🌐 The number-one law of the shithouse rat manager is when you cock up, the first thing you do is cover your tracks.
2005		B. Hare Urban Grimshaw 278: The police and the Crown Prosecution Service had cocked up his papers.
2018		G. Gilmore Base Nature [ebook] ‘What d’you bring the big cheese in for?’ [...] ‘In case we cock things up’.