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".
de Matt, Jeff, Chad 22 septembre 2007
1 more definition
Referring to children you've had outside your marriage with other women.
Got any side kids? Nah just the one with my wife
de Toolz 12 septembre 2007