Apparently, the derivation of grass is something the experts argue amongst themselves.
“Grass” was used as often as “mum” on New Tricks, for what it’s worth.
Interesting, thank you!
I was thinking perhaps it was rhyming slang for “Judas,” which might be pronounced “Ju-dass.”
“I ain’t no Ju-dass!” —> “I ain’t no grass!”
Plausible, I think.
I always assumed it came from “whispering grass”, but maybe that’s too recent.
There was some confusion when American audiences were watching Bodyguard and thought Rob Stark was banging his mom.
I believe I first ran into the term in The Inbetweeners. It was pretty easy to figure out in context.
I was confused at first when my friend from Newcastle told me he was going to “knock up” his girlfriend when he got off work.
Time was, knocking up was a recognised trade.
From “The Adventure of the Speckled Band”:
A lot of knocking up going on in those days!
Well, with all the “ejaculating” they did, are you really surprised?
Not really !
Judas is pronounced more Jude-us (rhymes with . er … “include us”)
Grass rhymes with either “parse” (S England) or “mass” (N England)
“Mass” is pronounced like … ? 
Er, yes. That’s the problem !
I’ve only ever heard one pronouncement (is that the word ?) of “mass”
whereas “pass” ,for example, could be pronounced 2 ways (for N & S)
According to the Cambridge Dictionary, it’s /mæs/ in both American and British English.