An abbreviation meaning "know your scripture." It is commonly used in online debates over theology. Popularized by cage-stage Calvinists, but has since seen wide usage among many Christian sects.
Monergist: The Bible makes it clear that we play no role in our salvation; It is only according to the will of God. You should kys.
Synergist. No, you should kys. According to Scripture, we can choose to accept or reject salvation with our free will.