What does the A in F/A signify?
"A" for "attack", as opposed to "F" for fighter.
You can sometimes get sort of silly arguments going in the "fighter pilot" community vs. the "attack pilots". The US Navy F-4 pilots counted themselves as "fighter pilots", flying mostly air-to-air missions to shoot down MIG's, to differentiate themselves from the A-4, A-6, and A-7 "attack pilots" who mostly dropped bombs on air-to-ground missions. The US Navy F-4 pilots seemed to think most of us flying F-4's in the USAF were "attack pilots" (or even "bomber pilots") because we spent most of our time doing air-to-ground missions vs. air-to-air missions, but we thought of ourselves as "fighter pilots". Even when I flew A-10's and F-111's, we called ourselves "fighter pilots", although we were 100 percent dealing in air-to-ground missions there.
We had a US Navy exchange pilot flying F-4's with us in one of my USAF squadrons in 1980. He was getting ready to PCS, and he wanted to get back to F-14's vs. going to the then new F/A-18. He wanted to be sure he'd be a "fighter pilot"!
"Human existence is based upon two pillars: Compassion and knowledge. Compassion without knowledge is ineffective; Knowledge without compassion is inhuman." Victor Weisskopf.