in the groove

Originally slang that first appeared in the mid-19th century with a (usually pejorative) reference to the difficulty of leaving a well-worn rut (see in a rut). As back in the groove, the phrase acquired a positive sense of returning to one's usual self after a period of illness, setbacks, etc. With special regard for music, originally US jazz slang from the 1920s, possibly with reference to the grooves of early records.

in the groove

1. (colloquial) Running or performing extremely smoothly.
2. (music, slang) Playing perfectly, perfectly in sync with others, or with perfect focus.
   • 1933 Aug., Fortune, p. 90:

     The jazz musicians gave no grandstand performances; they simply got a great burn from playing in the groove.

3. Used other than figuratively or idiomatically: see in,‎ groove.
   • 1869, J.E.T. Rogers's preface to Adam Smith's Wealth of Nations, Vol. I, p. 27:

     The whole course of legislation... had flowed in the same groove for centuries.

   • 1932 Oct., Melody Maker, p. 836:

     […] having such a wonderful time which puts me in a groove […]

   • 2005, Joe McCabe, Hanging Out With the Dream King: Conversations with Neil Gaiman, page 77:

     There's a time in everybody's life when they know precisely what is right. And when I was doing that, when I was in the groove, I knew exactly what was right.

• (music): in the pocket, in the zone

• groovy

• “groove, n.”, in OED Online ⁠, Oxford: Oxford University Press, 2020.