Kant affirmed therein that lying, as a general rule, is always wrong. Constant contradicted this idea, asking : how should you manage the case when someone comes to you and ask for protection from some bandits about to kill him, then? Shouldn't you tell the truth to the violent mob, that the guy they're looking for is indeed hidden in your closet?
Kant replies that the right thing to do is to lie to protect the innocent, but knowing that it's as a constrained exception to the absolute rule : "you should not lie". The fact that you commit a wrong for a greater good doesn't entirely excuse the wrong or alter the rule.
Most people, however, fail to adhere to this subtle analysis. Hence their displeasure with the word "dishonest".