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1.
A smile that forms when you have just gotten done smoking marijana.
Yesterday, I could tell Jim had an Illegal Smile
by AlexLemoine November 06, 2010
 
2.
The involuntary smile that forms when you know you've done something illegal
'I see that illegal smile on your face. What did you do?'
'Last night I blew up the neighbor's garbage can..and the curb'
by WolfRyder July 05, 2012