When we say we are going to "drop by" someone's house, we usually mean that we are going to pay a brief and informal visit. However, there are other ways to express this idea using different synonyms. For instance, we can say we are going to "pop in", "stop over", "swing by", "call in", "show up", "visit briefly", "make a quick stop", "pay a flying visit", "pass by", "run in", "come over", "check on", or "pay a surprise visit". All of these expressions convey the same general meaning of stopping by for a brief and informal visit.