As business process management is increasingly applied in practice, more companies document their operations in the form of process models. Since users require descriptions of one process on various levels of detail, there are often multiple models created for the same process. Business process model abstraction emerged as a technique reducing the number of models to be stored: given a detailed process model, business process model abstraction delivers abstract representations for the same process. A key problem in many abstraction scenarios is the transition from detailed activities in the initial model to coarse-grained activities in the abstract model. This transition is realized by an aggregation operation clustering multiple activities to a single one. So far, humans decide on how to aggregate, which is expensive. This paper presents a semi-automated approach to activity aggregation that reduces the human effort significantly. The approach takes advantage of an activity meronymy relation, i.e., part-of relation defined between activities. The approach is semi-automated, as it proposes sets of meaningful aggregations, while the user still decides. The approach is evaluated by a real-world use case.