"Side Yat" This definition comes from the saying Side man
Meaning: An irrelevant and powerless guy, that shows signs of moist
However with 'Side yat'. This is a girl to which a male counter part has some sort of relationship with, however she is more a piece on the side, or a back up if you will, incase his first yat
of choice decides to leave him.
This 'Side yat' does not really mean anything to the man, and is just used as a "little bit on the side".