Green’s Dictionary of Slang

1. a police officer [= copper n. (3); note WWI milit. grasshopper, a military policeman].

1921		N&Q 12 Ser. IX 344: Grasshopper. Policeman.
1938		F.D. Sharpe Sharpe of the Flying Squad 330: grasshopper : A policeman.
1943		J. Phelan Letters from the Big House 20: Copper is colloquial for policeman; grass-hopper is rhyming-slang for copper.
1969		S.T. Kendall Up the Frog.
1971		J. Jones Rhy. Cockney Sl.
1989		P. Manning ‘Sl.’ in Kray (1989) 62: Grasshopper is a copper: thus the word grass, / Which brings us to the prostitute and she is a brass.
2003		(con. 1932) W. Woodruff Beyond Nab End 29: Whenever a ‘grasshopper’ (a policeman) went past my window, the odds were he was on his way to the Nicholls.

2. an informer [= shopper n.].

1930		Eve. Herald (Dublin) 9 Dec. 4/6: There are many terms used by crooks to describe this person [i.e. an informer] who is known as ‘copper’s nark,’ ‘squeaker,’ and ‘grasshopper’.
1941		V. Davis Phenomena in Crime 254: A stoolie, Noah’s ark, a grasshopper. A nark or informer.
1956		B. Behan Quare Fellow (1960) Act II: I’ve never been a grasshopper or a nark for the screws anyway.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.
1982		G. Newbold Big Huey 73: Anybody who went to the screws [...] was labelled a nark, a grasshopper, or a policeman.