@awnlee jawking
'Fuck Around' I understand but where does 'Find Out' fit in? Is it a threat? Is it divination?
It may be used as a threat. However, for at least thirty, if not more than forty years it has been used as a warning.
Perhaps the person who utters is implying they will be the source of retribution. More often it was: "If you Fuck Around with those explosives like that you will Find Out why you were warned to handle them in a safe manner!" (FAFO is an acronym that could be used implying the above, because if you saw someone behaving foolishly with explosive you understood the context; but in a Forum Thread I need to describe the situation being addressed.)
In addition to multiple instances of foolishness involving 'Demo' (Demolitions aka Explosives), I have heard FAFO used since the mid 1980's.
Some examples: Staff NCO was cutting an Artillery Simulator (waterproof cardboard cylinder containing "black powder" granular explosives to make a loud Boom and a Flash; using a grenade type fuse), after cutting the device in half he used his lighter to ignite the "powder" {broke his arm, and all the hair on his face including his moustache and eyebrows were burned off} That incident became an US Army wide "Safe-T-Gram"
Our Platoon Sergeant had warned FAFO when we saw him sawing on the device. We had continued on our mission, and didn't observe him igniting the device.
We saw a "Duce and a Half" (2 1/2 ton truck) that the cargo bed had been heavily damaged, partially blown out by the right side of the cab, with damage to the cab too. We were told that several 40mm grenades (such as used in an M203 or M79) had been left loose in the cargo bed. They arm by spinning; that is the safety, they have a contact fuse on the nose.
We didn't know for sure that it was an M203 grenade that had caused the damage, nor if it had armed by rolling around. We had been taught that an M203 grenade arms by spinning, to prevent premature detonation, if the projectile might pass through heavy vegetation, or go wild and impact a tree soon after firing. (Not that we completely trusted such a safety to always function.) But when some idiot started throwing a "live" (not a training "blue powder) M203 grenade up, spinning, and catching it; an NCO growled, FAFO! Causing the idiot to stop.
FAFO was used in context of:
Choosing a foolish bivouac (camping) site (prone to flooding) they set up their Battalion where assigned, and when it rained heavily during the night suffered dozens of casualties, thankfully no fatalities, and hundreds of thousands of dollars in equipment damage. (This had occurred numerous times over decades; admittedly the site has been used hundreds of times with no problems.) Our unit had been assigned to a section of that Bivouac Area, but our chain of command used their initiative, and we bivouacked elsewhere (on higher ground). One of our senior NCOs had suggested to a Sergeant Major of the other unit that they displace, or FAFO.
Foolish Driving.
Foolish behavior on a Rappelling Tower.
Various activities relating to Parachuting.
Provoking someone no prudent person would...
Etc.
FAFO has been used in the US armed forces for decades in the late 20th and early 21st centuries.