وَالْغَرَابَةُ كَوْنُ الْكَلِمَةِ وَحْشِيَّةً غَيْرَ ظَاهِرَةِ الْمَعْنَى وَلَا مَأْنُوسَةِ الِاسْتِعْمَالِ
Gharaba is a word's being strange, its meaning not readily apparent and its use unfamiliar.
Gharaba concerns access to meaning and familiarity of established usage. It does not necessarily entail harsh pronunciation: a word may be easy to say yet so obscure that the intended sense is not readily recognized.
The fact that a term is rare does not by itself settle every case. The issue is whether the specific usage is clear within reliable Arabic linguistic practice.