ASS TO FLOOR.
Although [in most cases] it is not meant to be taken seriously, all it means is that you better go as far down as you possibily can. Can also be written as 'ATG' which would translate to 'Ass to Grass.'
EDIT: If anyone cares - http://www.wannabebigforums.com/showthread.php?t=75458
Originally Posted by Celeste