I was reading a dictionary and found such a description of the word "rusty": "If a skill you had is rusty, it is not as good as it was because you have not it". And I just can't wrap my mind around the construction of the final phrase "because you have not it". It means "because you don't have it"?.. Why is it written in such unusual way? Honestly, if I wrote so, my university English teacher would point it as a mistake. I can understand "have not" only if it was a part of Present Perfect tense, or if it was "You haven't got" like "You don't have", but how can it be "you have not it?"