Side chick is a slang term, most encountered especially in African American Vernacular English, though thanks to the increased prominence of hip hop and black culture since the late 90’s and early 2000’s, it is widely recognized in the Anglosphere.
A “side chick” is a backup plan, or a plan B, a guy is meeting and hooking up with as an alternative to the so-called main chick.
Alternatively, a “side chick” may be encountered as side bitch, though this use is more applied for its pejorative load as an insult.
It is unclear who, when and where came up with the term, as men are historically known to be prone to polygamy.
The word didn’t appear until the second half of the 20th century, however and was brought into common use, through the high amounts of reference in hip hop music, in the final years of the 90’s.
“Side chick” didn’t truly take off until the era of the internet, when it was featured and used in several memes, vines and YouTube videos.
The first entry on the subject was uploaded to Urban Dictionary in 2007, and several other definitions had been written since.