to squint, what? huh? (Cant.)
乜 centers on a narrowed or questioning gaze: squinting, or expressing confusion with a 'what?' or 'huh?'.
The historical origin of 乜 is uncertain. It may have developed as a variant or simplification of a more complex character, but no clear etymology is established.
The shape 乜 looks like a face with one eye squinting and a small questioning mark beside it: the curved stroke is the squinting eye, and the short right stroke is the 'huh?'.
For バ, imagine someone squinting and saying 'Bah?' in confusion: Bah -> バ, matching the questioning 'huh?' meaning.