This was overheard in a check-out line conversation, and is a great piece of “advice”. It is a nice mashup of “people who live in glass houses shouldn’t throw stones” (you should not criticize other people who have the same faults as you have) and “flog (or beat) a dead horse” (to insist on talking about something that no one is interested in). It’s a classic malaphor, incorporating two idioms to make a strange statement. Horses and houses seem to be the culprit here. A big thanks to Jeremy of Smoke-Eaters Studio for hearing this one and sending it in!