You know how to beat a dead horse in the mouth

Another horse malaphor.  This one is a mashup of “beat a dead horse” (to continue to focus or talk about something) and I think “don’t look a gift horse in the mouth” (if you receive a gift, accept it graciously).  “Horse” is the common denominator here.  “Shoot off (one’s) mouth” or “diarrhea of the mouth” could also be in the mix, both meaing to be an excessive talker.  That fits with “beat a dead horse”.

By the way, idioms that include the word “horse” are for some reason continually mixed up.  See my website and type in “horse”.  You will be amazed.  A big thanks to Thomas Smith for sending this one in.