To hate grind is to channel all the frustration you have into grinding something. Typically when you hate grind, you do it for yourself but also do it in spite of the people that don't believe in you.
"Did you see? This dude just laughed at Kariyu for being a bad player at osu!, and Kariyu FC'ed his top play in one try!"
"LMAO he's hate grinding"
When you grind in a game you hate, usually complaining about how unfun the game is and grinding out of pure hatred
"Ugh I fucking hate this game!"
"Then why are you playing it for 10 hours a day?"
"I am hate grinding!"