Surprisingly, it doesn't begin with an "f", and it's not slang for a female body part.
It's the word "actually".
The definition for this word should be "backhanded compliment". Because that's all it really is.
Let me explain further with some real-life examples:
KK: "What do you think?"
COWORKER: "It's actually a good idea."
Translation: "Wow, I like that idea. And I am shocked that you came up with it. Usually your ideas are crap."
KK: "This is the dress I bought."
MOM: "That actually looks good on you."
Translation: "Normally that color would totally wash you out and the empire waist would make your hips look big, but for some reason it's working."
Actually, I thought this post would be better.