An informer, police informer; one who betrays a group (of criminals, etc) to the authorities.
“What just happened must remain secret. Don't be a 'grass.”
Any plant of the family Poaceae, characterized by leaves that arise from nodes in the stem and leaf bases that wrap around the stem, especially those grown as ground cover rather than for grain.
verb
To lay out on the grass; to knock down (an opponent etc.).
To act as a grass or informer, to betray; to report on (criminals etc) to the authorities.