Kafir means one who rejects.
‘Kafir’ is derived from the word ‘kufr’, which means to conceal or to reject. In Islamic terminology, ‘Kafir’ means one who conceals or rejects the truth of Islam and a person who rejects Islam is in English called a ‘non-Muslim’. So, Kafir means non-muslim which is not an abuse at all.
For example, if a Hindu calls a Muslim a non-hindu, is it an abuse to that muslim person? Surely not. Similar for all other religions.
So, if a ‘non-Muslim’ considers being called a ‘non-Muslim’ or ‘Kafir’, which are one and the same, an abuse, it is due to his misunderstanding about Islam. He or she needs to reach out to proper sources of understanding Islam and Islamic terminology. Then he or she will not feel abused but appreciate Islam in the proper perspective.