'Thanks', 'okay', 'good' — 'safe' — as the London slang word that does the job of five, and the record's entry for the acknowledgement that means everything is fine. 'Safe, bro' ends a transaction; 'he's safe' is a character reference.
The record notes 'safe' is the London 'np', that it has been said to the record by a bus driver, and that the bus was, in fact, safe.
I saved a screenshot of the first Safe post I saw. Looking at it now, the joke is entirely in the timing. The timing does not survive the screenshot.
What does it mean? Write it the way you would say it out loud.
