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1.
If a man is married with a family, and has a child with another woman while still married. The bastard child is the the "side kid".
Autumn Jackson would be Bill Cosby's "side kid".
by Matt, Jeff, Chad September 22, 2007
 
2.
Referring to children you've had outside your marriage with other women.
Got any side kids? Nah just the one with my wife
by Toolz September 12, 2007