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1.
to "borrow" a cigarette from someone.
"hey bro can i bum a smoke from ya?"

"why certainly, good sir, you may indeed have a cigarette this fine evening."

"sweet thanks weirdo."
by silly hats only February 18, 2009
 
2.
to get a cigarette from someone for free.
Girl 1: hey can i bum a smoke?
Girl 2: sure. here, have fun getting cancer
by Kennedi Greene October 24, 2008
 
3.
To insert a lit cigarette in one's anus, and then fart with the intention of lighting the fart. Most often done with the lit end facing out, but can done lit end inside.

Originally a hazing ritual performed by British headmasters on boarding school students.
"Say, mate, can I bum a smoke?"
by A Nussbaum April 17, 2007