I bet he was sorry after the incident and acknowledged later that the words were inappropriate. If so, your dealing with lack of inpulse control (what's on his mind is on his tongue)with little control to keep it their. This is not offering an excuse just explaining the mechanics behind an ADHD kid's thinking. My son is the same with the word "NO" triggering his explosions. Depending upon the time of day, if we deny him something, we become the enemy and evil so that words and actions against us are justified. Just remember, not to get caught up in his energy and if you do, don't model the words you don't want him to say. Yea, I know it's easy to say these things but hard to follow through in action. Let him know that 'the family does not talk to each other that way' and then ignore him until he is in control enough to temper his language.
Paul
Just give him an automatic timeout every single time he does it. He'll last a week or so.