Definitions by SuperAwesomeBob

Sai 

vulgar, colloquial- An illicit lover

Oh he's got a wife but he's boning a Sai on the side

Sai by SuperAwesomeBob December 10, 2011

Sai 

Colloquial, in other countries- A lover who has to court in secret, generally female, due to social factors

He and his Sai had no choice but to make passioante love behind the bushes in Cubbon Park

Sai by SuperAwesomeBob December 10, 2011