Very good or pleasing. Superlative of Good.
It was grotten - It was very good.
She/He was/is grotten - She/He was/is very good fun.
Dirty, miserable, filthy, disgusting and generally wrong. Combination of grotty and rotten - when one word is not enough to describe the skankiness.
"Oh man, I woke up sweating in a tent after a 24 hour drinking binge. I feel grotten"