0 to understand a subject because of your experience: --
He doesn't know what he's talking about - he's never even been to Africa.
1 to be able to speak in an informed way because of your experience or education: --
He doesn’t know what he’s talking about – he’s never even been to Alaska.
NYA ORD
European
May 10, 2021
DAGENS ORD
Shimmer
know what you are talking about