used to indicate something is wrong, without anyone else knowing, (its the word 'cover' backwards). Usually when talking about a third party and the third party turns up and you need to change the subject quickly,or when you are unable to say something because someone else is there, works well on the phone.
<on the phone>
bod1: Johns a right tosser
bod2: um, i agree revoc, hes great..
bod1:you cant talk? ok.. buts hes still a tosser, call you later....