Stop teasing him about his new haircut, or you'll really annoy him.

teasing 是动词 tease 的现在分词形式。tease 的意思是"取笑、戏弄"，指通过言语或行为故意逗弄或嘲笑他人。现在分词 teasing 在这里表示持续进行的取笑行为。