I always thought that the name JSON5 pretends to be nothing more than a pun on Michael Jackson's original band, The Jackson 5. It sounds too similar!
If it didn't originate from that, what else?
Ha. There is no way that’s what it means unless you read that somewhere.
I agree with the others that ES5 is likely where it came from, but my mind also jumped to The Jackson 5.
I assumed it was a play on HTML5 ¯\_(ツ)_/¯
The project description describes it as json plus syntax from emcascript 5.1 (commonly called ES5) [the official name of javascript is emcascript]. I imagine that is where the name comes from.
Although it doesn't really make sense since most of the stuff they add predates ES5.