to peep, to spy
瞯 centers on observing secretly: peeping or spying, using the eye to watch without being noticed.
瞯 combines 目 (eye) with 閒, which likely contributes the sound and the idea of a gap or interval, suggesting looking through a crack.
The eye 目 peers through the gap of a gate 門 with a moon 月 inside, like spying through a crack at night.
For カン, imagine a can with a tiny hole: you peep through the can to spy on what's inside. Can -> カン.