Halsey is not letting Grizzlies player Desmond Bane get away with coming at the King ... ripping the guard for being rude to LeBron James and having a "big a** f***in' mouth."
23-year-old Bane was clearly chirping throughout the Lakers' home game against Memphis ... and it got so heated at one point, LBJ shoved the guy for continuously running his mouth.
The award-winning singer -- who's a die-hard Lakers fan -- was sitting courtside through it all .