1. Even though it's real meaning is "fucking ass", it is mostly used when telling someone "Damn right"
2. It is also used to tell someone that you did not just get what they said, but you're pretending to get it. This would be used with a questioning tone of voice.
3. Also, it can be used to diss someone. In this case, "effing a" would mean "fucking asshole". While saying this, it could aslo mean "what the hell" at the same time.
4. The term really started being used after it was on the movie "Dodgeball".
1. Girl,"I want you to fuck me."
2. Girl,"I was so like, oh my gosh, awesome at that thing a couple days ago and like it was so just ya."
3. Guy 1,"haha, I did your girlfriend last night hommie."
Guy 2,"dude, effing a."
4. Cotton,"That was sure amazing!"
Pepper,"Effing a Cotton, effing a!"