Forward Air Controller, sort of a FOO for planes but much more handsome and gifted. It is generally agreed that the FAC is the hardest bloke on the battlefield.
In theory he's there to direct friendly aircover to where the opposition is in most dire need of a kicking
AT LEAST keep kill crazy A-10 pilots from buttfucking our chaps because they think <insert stupid notion here>.
The FAC actually spends most of his time being told how to do his job by people who don't know what they are on about and annoying the rest of the Army by turning up on exercise in a civvie 4x4 then p1ssing off after the Air has gone.
Contrary to what a Foo will tell you, the FAC is the one who will de-conflict the Air with any other Joint Fires. Due to this he is the king of the FST.
Also how Southerners pronounce the British Army's all-purpose swear word.