According to Stephen King in Dreamcatcher, fuckaroo means an extremely fucked up situtation, as opposed to fuckaree which means having a great time, usually but not necessarily, sexual.
Fuckaroo is the sex act when someone gets on their hands and knees naked while another person gets on them naked and the person on bottom bucks as hard as they can to knock the other off and if they fall the one at bottom gets to have his or hers way with them