in (or into) the groove

1 performing well or confidently, especially in an established pattern.

2 indulging in relaxed and spontaneous enjoyment, especially dancing. informal

☞ A groove is the spiral track cut in a gramophone record that forms the path for the needle. In the groove is first found in the mid 20th century, in the context of jazz, and it gave rise to the adjective groovy, which initially meant 'playing or able to play jazz or similar music well'.