<2013 “‘Are we going to get in trouble?’ ‘Only if you grass me up Cross and Burn by Val McdDermid, page 97>
CASSELL’S DICTIONARY OF SLANG
GRASS (also GRASS SOMEONE UP) [1930s and still in use]: To inform, to tell tales, to betray.
GRASS British informal: Inform (or one who informs) the police of someone’s criminal activities or plans; a police informer [Perhaps related to the 19th-century rhyming slang grasshopper 'copper'] [[I like it!]]
Someone had grassed on the thieves.
She threatened to grass me up.
To find out who grassed on him read the rest of the review.
Somebody must be proud of having grassed on her.
The following quotes are from archived sources:
_______________________<1997 “‘What? Who says I've got a habit? You're trying to grass me up. It's too early in the day to do any of that.’ White has repeatedly denied taking drugs”—Sunday Mirror (London). 16 February>
<2007 “It will act more as a deterrent, the fear of whether my mate will grass me up.”—Daily Mail (London), 3 February>
<2009 “Hickman instinctively put up his arm to protect himself and he was hit with the sword by Hales who told him: ‘If you grass me up I will kill you.’”— Birmingham Mail (England), 3 November>
<2012 “These days Jesse’s old enough to grass me up to Angela.”—The Mirror (London), 30 March>
<2015 “‘No one does that. Grass me up and your dead, Danny-Boy.’”—Skarrs by Cathetine Forde>
Ken G – January 10, 2016 (Grass me up in Colorado – it’s legal)