Greg still insists that he did nothing wrong.

insists 是动词 insist 的第三人称单数现在时形式。insist 由前缀 in-（表示"加强"）和词根 sist（来自拉丁语，意为"站立"）组成，字面意思是"坚定地站立"，引申为"坚持、坚决主张"的意思。