In Cornwall in the South West of England, this is a dialect name for a woodlouse, so although not strictly slang it still owes its origins to dialect and common usage as slang words do. The difference between dialect and slang is that dialect is not constant, for example just across the Tamar in the county of Devon a woodlouse is ‘chickypig’ while in Somerset it’s ‘gramfy-coocher’ and in Gloucestershire ‘johnny-grump’.
Chagging is the portmanteau (combination) of choking and gagging. It is usually accompanied by an embarrassing loud, wet sound. Chagging has been referenced by Garfunkel and Oates in their "BJ Song."