The term "spankable-chest" is a combination of two terms commonly used in adult content: "spankable" and "chest."
Spankable refers to the idea that someone's buttocks or another body part is suitable for spanking during sexual play. In this context, it suggests that the chest is an appropriate area for spanking.
Chest generally denotes the area of the human body between the shoulders and the abdomen. So, "spankable-chest" means a chest that can be spanked as part of sexual activity or play. This tag might be used to describe a video in which someone's chest is being spanked or slapped as an erotic act.