Stacks up is a phrase that typically means "to accumulate" or "to compare or evaluate." Its antonyms can vary depending on the context in which it's used. For example, if it refers to a physical pile of objects, some possible antonyms could be "scattered," "dispersed," or "disorganized." If it's used to compare or evaluate someone or something, antonyms could include "falls short," "disappoints," or "underperforms." Additionally, if it's used in a positive context, antonyms could include "exceeds," "surpasses," or "improves upon." Ultimately, the most appropriate antonyms for stacks up will depend on the specific usage and meaning of the phrase.