It stands for "F(ool) Around and Find Out", understanding that the first word isn't "fool". The past tense is: "F(ool)ed Around and Found Out".
As a warning, it also implies that the person should accept responsibility for the consequences.
The best example of this was when I was walking through Intake, and spotted Waldo the Wonder Biker leaning against a corner of the holding tank, both eyes swollen shut, nose broken, and blood from his waist to the back of his head. Blinking a bit from the surprise I asked Waldo what happened.
Hocking a bloody lump the size of a kiwi fruit in my general direction, he growled, "I [deleted] around and found out, LT."
Whole bunch of folks who have been fooling around in far-Left Democratic strongholds are taking their foolishness into less-Leftist areas ... and finding out.
The latest is in Kenosha, Wisconsin, and seems to be a pretty spectacular example of "Finding Out", and while I won't speculate on the legal details of the shooting, some of the pictures are probably going to wind up in trauma lectures.
Folks f(ool)ed around, they found out; and now the million-dollar questions are: 1) Will they take responsibility for their [deleted]ing around that led to the finding out? 2) Will everyone else learn from this, and modifying their future fooling around?
Survey says: Probably not.