When you've hit it so hard (got mash up on a superb mix of drink and drugs) and you're being sick and you shit yourself at the same time
This generated from a mis-spelling of the word seriously - thus a new word and definition was born.
Contributions from Disco, Curly, Joseph Von Sniffon and Kimberley the Cainer
'maaaan i was so fucked i went home and i was seriosuly'
'do you know that cocktail of absinthe and ketamine got me seriosuly'