In a crude, vulgar, or insensitive manner; without regard for propriety or other people's feelings.
From the adjective 'crass' with the adverbial suffix '-ly' added. The '-ly' suffix transforms adjectives into adverbs describing how something is done.
Adding '-ly' to 'crass' creates an adverb that describes the manner of behavior—'he crassly interrupted her' means his interruption was done in a rude way!
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