Popping in refers to a brief visit or dropping by somewhere for a short while. Some common synonyms for popping in include "stopping by," "visiting quickly," "making a brief appearance," "checking in," "calling in," and "dropping in for a moment." Other phrases that convey the same meaning include "swinging by," "nipping in," "popping over," and "running in." Regardless of which phrase you choose to use, the goal is to convey the idea of making a brief visit, without staying for an extended period of time. By using these terms, you can express your intent to stop by quickly without disrupting the other person's schedule.