To Fuck around and Find out. To learn by trying; getting one's hands dirty with direct experience; to do, not to read about; to jump in.
Sam needed to repair his lawnmower. Instead of reading the manual he simply F&F'ed until the issue was gone. Saving him time reading, as adding time to the repair.
Farts and fuzzles, an act of snuggling with your sister and sniffing each other in a bed whilst making each other warm enough to emit gas.
Hey sis, let's go for an f and f session!