:dancing-banana:
Maybe. It depends upon who is speaking to whom, about what, in which manner. Obviously it was not a sin for Our Lord to call the Pharisees hypocrites or blind leaders of the blind. Was it sinful of John the Baptist to call the Pharisees a brood of vipers? St. Paul DID apologise for calling the high priest a whited sepulchre. Was that a sin?
Our Lord warns us about calling another a fool, and that every word we speak will be accounted for at our judgement. In general, it is unbecoming of the Christian to insult others, but there are surely exceptions. St. John is said to have insulted a known heretic and refused to be in his presence. Is the intention to injure another's soul? If so, it is sin. The wise man inquires, "...a wounded spirit, who can bear it?" Is the intent to ruin another's good reputation by backbiting, detraction, calumny, slander? This is spiritual murder. Do we mock or scorn or reject God-ordained authority? Sin is not lacking. Do we spout put-downs and crude remarks to amuse others or raise our sorry egos by lowering others? Not good! Do we speak the truth though it be hard to hear that another's soul be converted? Though it backfire, it is not sin. Do we warn others by way of strong words when it is our duty? Again, not sinful, even if poorly-received. Perhaps a test is this. Ask if yourself if you would call another by a certain name to his face. Ask yourself what this would accomplish.
Cultural norms and social position must be taken into account. It is not sinful for two old men to jokingly "insult" one another. Both know it's a form of teasing. It is another matter for an employer to put a negative performance review on-line to publicly humiliate without necessity. Parents should not mock or scorn priests or religious in the hearing of their children, then lament their children's lack of respect for authority.
Does Bishop Williamson sin in referring to Bishop Fellay as a weasel? Somehow, I do not doubt he'd say it to his face. He doesn't personally hate Bp. Fellay, but he does hate his ambiguous and contradictory statements. Bp. Fellay could reply in like words because they are both on the same level, two bishops butting heads. If, as an ordinary lay woman, I walked up to Bp. Fellay and called him a weasel, it WOULD be sinful. It is not in keeping with my place to do so, and could indicate only disdain for his spiritual position. If I needed to convey displeasure, it must be done with respectful words. On the other hand, with a priest whom I know well from childhood, I might make a sarcastic comment to tease him, and he might do the same to me! If done at all, the context must be one where it will not be misunderstood and become an occasion of scandal to others. Of course, we don't need to be "nithe to evweewon!" Flattery can be a form of insult as easily as blatancy. What greater insult to Christ than for Judas to greet Him with a kiss. What could have been more generous than for Christ to then address Judas as "friend?"