In chess, a "thorn pawn" is a pawn which has reached the sixth rank and is blockaded from further movement by an opponent's pawn, especially if the pawn is somewhere in the vicinity of the opponent's king. Thorn pawns are often quite hard to dislodge, and can play a crucial role in limiting the movement of the enemy king and setting up a mating net, especially in combination with a queen.
They are called "thorn pawns" because they are extremely annoying, like a thorn in the opponent's side.