Stacked up is a term that refers to a pile or collection of items placed one on top of the other. There are several synonyms that can be used to replace this phrase such as piled up, heaped up, accumulated, amassed, gathered, arranged, and assembled. All these synonyms describe the same action of placing objects one on top of the other, creating a stack or a heap. These words are often used in the context of organizing and arranging items in an orderly manner. Whatever the context, using synonyms can help add variety to your writing and make your message more engaging.