Albert Barnes Commentary Judges 16:21

"And the Philistines laid hold on him, and put out his eyes; and they brought him down to Gaza, and bound him with fetters of brass; and he did grind in the prison-house." — Judges 16:21 (ASV)

Put out his eyes - Thus effectively, as they thought, preventing any future mischief on his part, while they prolonged their own triumph and revenge. (2 Kings 25:7; Jeremiah 39:7.)

They put brass fetters on his feet (2 Samuel 3:34; Jeremiah 52:11), and made him “grind”—the special task of slaves and captives (Exodus 11:5; Isaiah 47:2; Lamentations 5:13).