If you are unaccustomed to something, you do not know it very well or have not experienced it very often. They were unaccustomed to such military setbacks It is a part of Britain as yet largely unaccustomed to tourists. = unused accustomed
not habituated to; unfamiliar with; "unaccustomed to wearing suits" not customary or usual; "an unaccustomed pleasure"; "many varieties of unaccustomed foods"; "a new budget of unaccustomed austerity
If you describe someone's behaviour or experiences as unaccustomed, you mean that they do not usually behave like this or have experiences of this kind. He began to comfort me with such unaccustomed gentleness