F12

People have correctly identified the meaning of F12 as but no one has provided the actual answer as to why “12”

The phrase F12 is technically conveying F+12 other letters (u c k t h e p o l i c e) This was common on Twitter when tweets were limited to 160 characters.

Other instances where this mechanism was used - L10N = localization (L +10 letters +N)

“Dude, you shouldn’t say that the cops will hear you” “Man, F12”