The phrase "爷压在妈身上" is quite informal and may have different interpretations depending on the context. In a literal sense, it could refer to a situation where one person (referred to as "grandpa") is physically on top of another person (referred to as "mom"). However, this phrase might also be used metaphorically in certain contexts, such as describing a power dynamic or relationship status.

In English, a direct translation would not make much sense due to cultural differences, so it's important to consider the intended meaning. If you're looking for an appropriate translation based on context, please provide more details about the situation or relationship being described. For instance, if this is part of a story or dialogue, additional surrounding text can help clarify how best to express the idea in English.