Set over set occurs when both you and an opponent have sets of different ranks on the same board.
Explanation. For example, you hold 5‑5, flop comes 5‑K‑K; opponent holds K‑K—both sets on board but theirs is higher. The term captures the disappointment: you flopped a strong hand (“set”) only to find a bigger one. These cooler situations are rare but costly and emotionally impactful.