There are a bunch of proposed etymologies, including rhyming slang (grasshopper = copper), but none of them are very convincing.

Others include "snake in the grass" or the song "Whispering grass" (although the first use predates the song by 8 years).

It could also be rhyming island from "grass in the park" -> nark.

Grasshopper handily also explains "shopping" someone.