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

I couldn’t care less (but one must keep up appearances, right?)

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.

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.

A cockup means someone has made a real mess of something

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