be silent, be quiet
嘿 centers on silence and stillness: being quiet, not speaking, or keeping silent.
嘿 combines 口 (mouth) with 黒 (black), suggesting a mouth that is dark or closed, i.e., silent. The exact historical development is uncertain, but the component combination fits the meaning of silence.
A black (黒) mouth (口) is a mouth that is shut and dark, making no sound. Picture a mouth sealed with black tape to keep it silent.
For ボク, imagine a silent monk (ボク) with a black mouth, meditating in complete quiet. The monk's vow of silence ties the reading to the meaning.