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1.
Xum
An abbreviation of the word "Excuse Me." It gets its sound from combining the "X" sound in "Ex," the "U" sound from "c-UE-se" and of course the "M" from "Me."

It can also be used as a question, as in "Pardon me?"
Example 1:
*Someone sneezes*
Someone: "Xum."

Example 2:
A: "Does it smell like Up Dog in here?"
B: "Xum?"
by Allan Wood March 10, 2008
 
2.
xum
Like yum or awesome, used to describe something that is fun or cool.
Its pronounced "zum" First used in in the online comic strip "ALLAN"
JEN: Did you see "Donnie Darko?"
ALLAN: Yeah! It was really xum!
by Cm Bear February 03, 2008