Not common or usual: "The legislature has produced a new budget of unaccustomed austerity” ( People). (adjective)
Not being habituated. Used with to: is still unaccustomed to a life of stress. (adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "unaccustomed" in a sentence
"“Cicero must be really desperate,” he said, with an expression of unaccustomed seriousness, “to ask for help from me."
"Even now he recalled the unaccustomed world when he woke up, the strange feeling it had given him."
"Moreover, having very little idea of the lapse of time -- being quite unaccustomed, that is, to reckon events from any fixed era -- primitive man cannot have gained at once a clear conception of age as applied to his fellows."