My classmate Martin is very tactful but he has a great sense of humour.

tactful 是由名词 tact（机智、得体）加上形容词后缀 -ful（充满...的）构成的形容词，意思是"言行得体的、机智的"。tact 本身来自拉丁语，原指"触觉"，引申为"处事圆滑、善于把握分寸"。