When a smiley emoticon, :) , is used in an email or instant message immediately after something insulting or otherwise putting you down.
Typically the FU Smiley is a way for the sender to downplay their attack... but really, instead of being the happy-go-lucky smiley face that it usually is... the FU Smiley is really saying "F$#K YOU!"
You failed to show up for the meeting again... this is the sixth time in the last month. I cc'd your boss on this email just so you can be held accountable for where you need improvement. :) (Boom - there's the FU Smiley)