You explain it by pointing out a weakness of the employee who wishes to gossip about a co-worker, mentioning that all employees have weaknesses, including the cocky ones.Don't actually say that last part, however.After describing the employee's weakness in detail, tell him or her that you aren't recommending any disciplinary action in this case.Then, ask the employee how it would feel to be scrutinized for deficiencies in work performance with an audience of colleagues who were competing against each other at work, especially when the gap in performance (or other errant behavior) warranted disciplinary action.