Definition of grind
When we talk about the "grind" in poker, we are talking about the life of a regular poker player, a pro player who earns his living with poker. In the idea of grind, there is the idea of long and hard work, of constant effort, of relentlessness, of a small but steady payoff. A grinder is a player who "grinds", i.e. plays several tables simultaneously and plays long hours. A grinder is a player who works hard for his winnings. He or she is reliable, consistent, diligent and wins regularly.
The image of a grinder
The word "grinder" in English is a tool that grinds (like a coffee grinder, a tool that grinds coffee beans) or a tool that saws, but little by little. If we want to cut a very strong padlock, we can use a grinder, a tool that will cut the metal efficiently, but slowly. Grind necessarily has a pejorative connotation. The grinder is not a poker millionaire, he is a player who pays his bills with his poker, but he will have modest winnings that he will accumulate quietly. The grinder is still a solid player. He is considered a serious and winning player.