Oddbean new post about login | logout https://wordhistories.net/2020/02/15/not-have-scooby/ (Heard the word on Sherwood which is turning out to be pretty good. Generally speaking, UK TV is a trillion miles beyond North American.)
There’s also a side meaning that’s drug related ( https://www.urbandictionary.com/define.php?term=scoobie%20snacks ) but generally it does just refer to not having a clue.