"To spoil an otherwise successful endeavor or to ruin an otherwise positive moment." A sort of haimish or heimish (i.e., Jewish-inflected) version of "turd in a punch bowl" or "turd in one's drink." A mikveh is a ritual bath used for purification and conversion in the Jewish tradition and, thus, shitting in one is strictly forbidden.
Me and my buddy were having a great morning shooting out on Forest Service land until a couple of noobs with zerotarget acquisition skills and good sense showed up and took a massive shit in the mikveh by sending a bunch of 5.56 across our baffles.
The one, that guy everyone wants to be like. The man next door who you can count on. A true ladies man, a modern pimp. A beast in the bed room, and a friend to the end.