stare at
瞪 centers on an intense, fixed gaze: staring at something or someone with wide-open eyes.
瞪 combines 目 (eye) with 登, which likely contributes the sound and may suggest a fixed, upward gaze. The exact historical development is uncertain.
The eye 目 on the left and 登 (climb/ascend) on the right suggest eyes that are wide open and fixed upward, as if staring intently at something climbing or rising.
For トウ, imagine staring so hard your eyes feel like they're going to pop out: 'toe' -> トウ, as if your eyes are bulging like big toes.