The phrase has indeterminate meaning and unclear origins.
Usage varies. It can mean cool; inebriated; "jacked" on stimulants; or "high" on benzodiazepines.
It can also mean all or none of these things. Proper usage varies with time, place, and context. However, the specific meaning intended is always easily conveyed, due to the common usage of the colloquialism.
Dave: What are you doing dude?
Berg: I'm just scoobying over here man.
Dave: Cool, I'll come over and we'll sort those bolts together.
Jack: What are you cooking Alex?
Alex: I'm making hotdogs.
Jack: That's so scooby.