Explanation of Grinding
Grinding carries the idea of something that is dull and repetitive. Poker can sometimes feel that way after playing for long hours which is how the term became linked to playing poker.
The term doesn’t always have to carry a negative connotation however, since in the modern world poker players use the term simply to mean “playing poker”.
Example of Grinding used in a sentence -> I wake up around 10, take a long shower, then immediately start grinding (playing poker).