absolute beginner; complete amateur
Emphatic phrase stressing total lack of experience or skill. Often used self-deprecatingly or to describe someone clearly out of their depth.
ずぶの素人なのに、いきなりプロの大会に出るなんて無茶だ。
It's reckless to suddenly enter a professional tournament when you're a complete amateur.
彼は料理に関してはずぶの素人で、包丁すらまともに握れない。
He's an absolute beginner when it comes to cooking; he can't even hold a kitchen knife properly.
The prefix ずぶ is an intensifier meaning 'completely' or 'thoroughly', often used in phrases like ずぶ濡れ (soaking wet). Its exact origin is uncertain, but it may be related to onomatopoeic or mimetic words. 素人 means 'amateur' or 'layperson'.