Performing very well, excellent; also, in fashion, up-to-date. For example, The band was slowly getting in the groove, or To be in the groove this year you'll have to get a fake fur coat. This idiom originally alluded to running accurately in a channel, or groove. It was taken up by jazz musicians in the 1920s and later began to be used more loosely. A variant, back in the groove, means “returning to one's old self,” as in He was very ill but now he's back in the groove. [Slang; mid-1800s]