saying "dead as fuck" means youre at an emotional coma. basically its when you get to a point of laughter or shock that you really have nothing else you can do or say, so you just kinda sit there lifeless - or dead. mostly used on the internet but it works around friends too.
*your friend falls down a huge ass set of stairs yelling the whole time*
you: (assuming theyre ok) bwahahahahahahahahhahahahahahahahahahahahah i am DEAD AS FUCK!!! you shoulda seen your face!