Green’s Dictionary of Slang

tale-telling, informing; thus grasshopper v., to inform on.

1953		B. Behan Scarperer (1966) 127: You want to keep a tight eye to that Lugs and see he doesn’t go grasshoppering round the place.
1958		(con. 1940s) B. Behan Borstal Boy 170: You grass-hopping bastard.