From 染み (shimi, "stain," "blot") + を + 付ける (tsukeru, "leave a mark on").
染(し)みを付(つ)ける • (shimi o tsukeru) ichidan (stem 染(し)みを付(つ)け (shimi o tsuke), past 染(し)みを付(つ)けた (shimi o tsuketa))