Used to describe something (eg. an all-nighter) a bit mad/crazy/wicked etc. for various reasons, usually relating to alcohol/drug intake.
"did you see how many bennie's kel did last night?!!"
"yes! he had such a fiendish one the nutter!"
Modern cockney slang, meaning:
It's all sweet
, everything's fine/good/well etc.
"Are you sure you're ok?"
"Yeah, it's all kooshtie mate."