A supergrass is an informer. The term originated in the UK where informers were often referred to as 'coppers', in relation to the help they provide to police with investigations - the Cockney rhyming slang for 'copper' is 'grasshopper'. This was eventually shortened to 'grass'. The word 'super' was later added to suggest a large-scale informer.
Hmmm.. learned something new and useless today!
LOL