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1.
An evil genius who has tried to kill; Bart Simpson; Selma Bouviere; Krusty the Klown (x2) and probably others. He has also attempted to blow up Springfield, but mended his ways twice, thrwarting his brothers plans to flood Springfield and helping homer find the man who was trying to kill him (Frank Grimes' son).
Sideshow Bob bears a grudge against the world.
by Mortenson January 30, 2004
 
2.
Only the most intelligent homicidial maniac ever. Created by Matt Groening... tried to kill Bart, Selma, Krusty... many people, actually. See above.
'AAAAAH! SIDESHOW BOB!' 'Oh, we've been through so much together... just call me Bob.' 'AAAAAAAAAAAAAH! BOB!'
by GizGaz August 05, 2005
 
3.
Named after the Simpsons character due to the resemblance to his hair, this is when a cigarette is rolled and the finished product has an excess afro of tobacco poking out of the top.
Haha, look at Lauren's fag everyone, she's got a major Sideshow Bob goin' on there.
by TheKHeroine March 21, 2011
 
4.
Sideshow Bob existed before the Simpsons, the expression is a generic term for any insignificant person. More likely to be used by a cheeky mate or a toxic friend rather than an enemy.
Who's that loser she's going out with, some sideshow Bob, no idea.
by mongrelbeast January 01, 2008