Grinder - Grinding it Out
1) A player player who puts in his poker hours in a job-like way
2) Grind it Out: working particularly hard, like when experiencing an unlucky run of cards, to make an acceptable return on your time invested; or when making a solid return when playing from a small bankroll at low limits
The term comes from the old brick & mortar casinos, when poker players play very long sessions they often need a stimulant to continue so they can stay up. The abuse of stimulants makes one grind their teeth, hence the term "grinder".
I've always taken the term as meaning monotonous, repetitive action over a long(ish) period of time. You hear it a lot in online RPG games as well as poker. "I'm grinding the orcs at the dark cave for gold today".
I would think it comes more the "Nose to the grindstone" expression than from stimulant use. If you're working hard you've got your nose to the grindstone. When it gets shortened, as everything does in modern language, you're just grinding.