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1.
Someone that wants something really bad - usually referring to drugs, but equally applicable to anything else.
That jonser is so damn hungry - give him some lettuce!

or

Look at that jonser - he needs some luvin.
by biggerP April 25, 2008
 
2.
a joke,or idiot, mess, or anything else negative referring to the person being a dumbass.
mike listens to screamo. what a jonser
by jacob wiskel August 07, 2006
 
3.
Someone who is always in need of drugs or never has cigarettes and always asks for one.
That dude is such a jonser, he's ran out of coke days ago.
by Jonesy602az April 21, 2008