in the groove: 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] http://www.answers.com/be%20in%20the%20groove performing well, especially performing well together http://en.wiktionary.org/wiki/in_the_groove adj. In the groove (1932) performing well. In a state of mind or mood conductive to playing music, esp. swing music, well; in rapport with the piece, esp. of swing music, being played. Orig. c1935 swing use, by musicians and devotees. Some resuurrected cool and far out use since c1955. Playing swing music intensely, with excitement, adroitly, in such a gratifying way as to elicit a strong response from the listeners; in rapport with or enraptured by the swing music being played. Common swing use late 1930's and early 1940's. When a phonograph plays, its stylus or needle is in the groove of the record (Archaic). http://www.phrases.org.uk/bulletin_board/26/messages/1110.html

doing it right, it feels good, like something has been done to perfection. Personally I use it to describe wiping my butt.