The phrase "faintest idea" means not having any knowledge or understanding about something. It refers to having no clue or any smallest amount of understanding about a particular subject or situation. Full definition
Also, you have no idea how its going to begin — and an even fainter idea of how its going to end. (health.com)
I had a faint idea of how to interact with potential customers, but no real - life experience doing so. (inc.com)
Sure, more people than ever have probably heard about bitcoin, and they may have heard about some of its closest rivals by market cap, such as Ethereum and Ripple, but they don't have the faintest idea what purpose they serve or how they really work. (fool.com)