notion
/ˈnō-shən/
noun
an individual's conception or impression of something known, experienced, or imagined
an inclusive general concept
a theory or belief held by a person or group
He had a notion to try skydiving.
idea concept conception thought notion impression mean what exists in the mind as a representation (as of something comprehended) or as a formulation (as of a plan). idea may apply to a mental image or formulation of something seen or known or imagined, to a pure abstraction, or to something assumed or vaguely sensed. concept may apply to the idea formed by consideration of instances of a species or genus or, more broadly, to any idea of what a thing ought to be.
idiom
to not know at all
He didn't have the faintest idea/notion what she was talking about.