The act of rushing backwards with your pants down to an unsuspecting person. Best used when your roomate is in his room attempting to make out with a girl. Get a group of guys together and burst through the door running backwards with everyone's pants dropped down. While charging butt first, proceed by yelling BUTT RUSH! Continue to charge the roomate until he starts yelling out threats that he may actually carry out.
Dude, last night was bogus. I was totally gonna get some until you all butt rushed me! Next time tell Will to wipe his ass atleast! I can't afford to throw away any more t-shirts.