It means to not do anything stupid and stay safe. It is definitely not an acronym for kys.
Ben: Get out of the road you asshole!
Dean: How about you keep yourself safe!
Ben: How 'bout you keep yourself safe!
By Estrellita
Keep Yourself Safe
people say this in order to inform someone to "kill them selves" without receiving any consequences from using the harsh term since both phrases are long for "KYS"
"bro this is to easy you're so shit"
"keep yourself safe dude"
