The state or quality of being accustomed to something; the condition of being familiar with something through habit.
From accustomed (adjective) plus -ness, a suffix that creates abstract nouns expressing states or qualities. This word structure is common in English for nominalization.
The -ness suffix is one of English's most productive tools for turning adjectives into abstract nouns—'accustomedness' captures that intangible feeling of 'I've done this so much it feels like part of me.'
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