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1.
A drink made by someone homeless, or soon to be homeless, combining at least 10+ different drinks that have been removed from a garbage can in a party district.

Also known amongst the homeless community as a "Sidewalk Sleeper", this is semi-common activity in Bourbon Street, New Orleans, LA.
The homeless guy made a Bum Runner on Bourbon Street as tourists laughed and photographed him in the act.
by Brandon "Batman" Green June 14, 2011
 
2.
A group of people who perform anal intercourse on an innocent girl, and then proceed to another innocent girl
Yesterday, my group of bum runners hit up four fat chicks at the club.
by DMedlock January 23, 2007
 
3.
A term used while paragliding on small practice hills where you are not able to actually take off, but still able to sit back in your harness so your bum drags along the ground to the bottom of the hill.
-How was the site?
-If the winds were strong, there could be some lift, but without decent wind we were only able to do a few bumrunners.
by Condor2008 April 13, 2009