extension of safe, used by hardcore welsh gangster rappers the GLC. meaning that is 'cool as hell', 'really rather fabulous' or can be used as a farewell.

Used by the ever growing Chav culture in England to describe an object, event or doing of something to be realy great. Often spelt Safe as, or Safe as fuk.
Alternatives are:

Off the box
Mint
cushtie

Perverted from the British slang safe, which meant everything from cool to "hello", depending upon context, "safe as fuck" is used to refer to being high, low, or stoned in any context, whether intentionally or not. There is an aspect of the term that is connected to just how out-of-touch the person using the term is; generally, if someone is high enough to use the term, they are only safe in their own minds and should not drive, operate machinery, etc.

use also as a greeting