fierce, ruthless

狠 is composed of the BuShou of犬(dog) and艮(the picture of a man twisting has neck and staring back). When someone is twisting his head and staring backward, his head is limited by his neck and cannot rotate for 360 degree. 艮 means extremity here, dog barks at stranger, seems so ferocious, so 狠 is used to describe someone whose personality or behaviour ferocious like dog.