a subject that regularly and over a period of time causes amusement whenever it is mentioned. Uncle Jim's driving was a standing joke. He used to drive incredibly slowly. Their mother's inability to make a decision was a standing joke in the smith family all their lives.
Something that is always funny even though it is often repeated. For example, Mary's "Dennis who?" when her husband is mentioned is a standing joke around here. This idiom employs standing in the sense of "established" or "regular," a usage dating from the mid-1500s.