他说我狠单纯

This nickname conveys a somewhat contradictory idea where 'he' (someone else) claims the person is very innocent or naive ('单纯'), even though it's unusual for someone to describe another person as 'hen' (very, which is likely meant as '狠' - fierce/hard, but might be a typo in this context). Here it implies a sense of irony or disbelief from the perspective of the person using this nickname.