a) A musical note, followed by G, preceeded by F.
b) A profane gesture. Join thumb and middle finger together, and gently extend index, ring, and pinky fingers. Can be used with or without the curse.
c) A curse. Similar to "Fuck you" though less intense. Usually used among friends in a joking manner, often accompanied by the gesture mentioned above.
"Hah. You can't hit a C, but I can!" *insert F# gesture here*
"TRUMPETS! F FLIPPIN SHARP!" *shake F# gesture*