In a manner that shows one is accustomed or habituated to something; as is one's usual custom.
From accustomed (adjective, past participle of accustom) plus -ly. The -ed ending creates an adjective, and -ly converts it to an adverb of manner.
This adverb is quite archaic but beautifully captures the idea of doing something with the ease of long habit—the way an expert musician plays without thinking, having done it so many times.
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