1
general::
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]
American Heritage Idioms
2
general::
Phrase(s): *in the groove
Sl. attuned to something. (*Typically: be ~; get ~.) • I was uncomfortable at first, but now I’m beginning to get in the groove. • Fred began to get in the groove, and things went more smoothly.
McGrawhill's American Idioms And Phrasal Verbs