Crassly is the adverb: you do something in a stupid and offensive way that shows you do not understand or care about other people's feelings. The meaning is the same as crass; this form just sits on the verb. Someone speaks crassly, jokes crassly, or crassly uses a private story to make money. It still has that thicker sense of tasteless and clumsy, not merely impolite. Like crass, it is more at home in careful speech and writing than in chat among friends. In ordinary talk, people are more likely to say they did it in a really rude way.