Something that's horribly screwed up, ruined or destroyed. Originally a British expression. Can be used as a noun or verb.

It’s an acronym standing for Can’t Remember A Fucking Thing. It’s usually used to describe embarrassment at being unable to remember someone (CRAFT moment); going onstage as the lead in the latest theatrical performance and your mind goes completely blank (CRAFT situation); the next morning after a night when you got pissed as a fart and woke up in bed cuddling a traffic cone and a string of fairy lights wound round your prick.

British slang for a mistake

A shambles. Sounds vulgar, but may not be; it may be a grouse shooting term. Customarily, shooters employ beaters to disturb birds on the ground and drive them into the air towards the guns. Normally one only shoots grouse hens, so if the beaters were by mistake to put up a grouse cock, they would warn the guns not to shoot by shouting "Don't shoot! It's a cock up!"

Can also be a transitive and intransitive verb.

A cockup means someone has made a real mess of something

To make a complete mess of things. Derived from the traditional archery usage where, if the 'cock' feather was uppermost, the arrow would not fly straight.

Another word for screw-up or screwup; to err.