In skateboarding or other roller sports, this term basically means doing an ollie (or jump) up onto the edge of objects like tables, rails, fun boxes, etc. with a skateboard and grinding it along the surface until you pop off of it back onto another surface or back onto the ground.
A dance performed (often in a club setting) in which the "grinder" places his hands on his head with his feet planted in the ground and proceeds to swivel vigorously
Such action can be stimulated by an exhilerating experience
Joske is grinding like a FIEND this evening! I've never seen anyone generate so many revolutions in one grinding session!
Bartholemu began to grind with uncanny resolve when he discovered that he had won tickets to the race.
Macker: "What are you folks up to this evening?"
Grinder: "We're in the midst of some unruly grinding. Wanna join?"
Macker: "I guess."
When you're trying to recruit Kecleon in Pokémon Mystery Dungeon: Explorers of Sky
My friend and I were grinding yesterday trying to recruit Kecleon in Pokémon Mystery Dungeon: Explorers of Sky while listening to Sea Shanties over Discord
When you work really hard and put a lot of time towards something. Used commonly in the context of achieving a goal or object of desire, whether that be in real life, a sport, a video game, etc.
Jeff: "Nice car dude, how could you afford that?"
Mike: "Man I just been grinding for it, been working hard at it. Spent a lot of time trying to get this thing."