Ithm in every Arabic dictionary would mean sin. Ghamidi sahab didn’t translate ithm in sura ey araaf on basis of its literal meanings in the dictionary but on the principle of tajreed. I am not sure what is the exact English alternative of Tajreed, but this is the concept in the language where a word is used in such a manner in a sentence that conjoining nouns strip a word of its expansive meaning and cuts it short to only remaining meaning.
For example in sura araaf God says:
قُلۡ اِنَّمَا حَرَّمَ رَبِّىَ الۡـفَوَاحِشَ مَا ظَهَرَ مِنۡهَا وَمَا بَطَنَ وَ الۡاِثۡمَ وَالۡبَـغۡىَ بِغَيۡرِ الۡحَـقِّ وَاَنۡ تُشۡرِكُوۡا بِاللّٰهِ مَا لَمۡ يُنَزِّلۡ بِهٖ سُلۡطٰنًا وَّاَنۡ تَقُوۡلُوۡا عَلَى اللّٰهِ مَا لَا تَعۡلَمُوۡنَ
Now here is how I understood it by Ghamidi sahab
When ithm is used alone it’s used in its expansive meaning, that is sin, but sin can be of only three kinds, 1. Obscenity (Fawahish), 2. usurping right of other (Haq talfi) and 3. Oppression or being unfair (Baghi baghayr il haq).
Now in this sentence ataf and matoof have stripped the word Ithm from the rest of its meaning and only one meaning is left in it.
So since the sentence has a rest of two, the only remaining mean is usurping the right of others or Haq talfi.
In English, if I say ” For tomorrow’s party I have all kinds of forks and knives, just bring the rest of the cutlery”, I am saying just bring spoons. The ataf and matoof of knives and forks had stripped cutlery of the rest of its meaning and cut it short (tajreed) to only one thing that is spoons.
Hope it makes sense!