Shackle Slinger is a uncommon Creature — Human Soldier card from Outlaws of Thunder Junction (OTJ), collector number 65. Current market price: $0.25 (foil $0.25), TCGplayer market prices via Scryfall.
Whenever you cast your second spell each turn, choose target creature an opponent controls. If it's tapped, put a stun counter on it. Otherwise, tap it. (If a permanent with a stun counter would become untapped, remove one from it instead.)