נראה שיעקב אבינו לא חשש שיוסף מת מדלא נראה אליו בשום פעם בחלום, ועל כרחך סבר שנטרף בדעתו והולך ושוכב בין החיות, כי חיה אחת ברדלס שמה שנשיכתה מטורפת דעת האדם, והיינו שאמר 'טרף טורף' שנטרף בדעתו, משום 'דלא ראיתיו עד הנה
I'll translate what he says and perhaps that will make it clearer.
It seems that Yaakov didn't believe that Yosef had actually died, as he never saw him (subsequently?) in a dream/vision. Rather, he held that it must be that Yosef became mentally ill (lit: his mind was torn up), and he lives amongst the animals. Since there's an animal called a jackal that through its bite makes people crazy (sounds like rabies). This is why it says the double expression טרוף טרף, as it means his mind was torn, since (Yaakov said) "I haven't seen him until now".