Onomatopoeic representation for the sound a primate makes when he flings bodily discharges (feces, urine, or semen) at unsuspecting onlookers. Typically used in reference to something disgusting that may or may not be flung through the air.
We each had a can of silly string for our silly string fight. As we sprayed the cans, we yelled "F'NA!" The silly string f'nah'd through the air. After the battle, we were all covered in f'na.