appear to have forgotten everything
嗒 describes a state of appearing to have forgotten everything, as if one's mind is blank or absent.
嗒 combines 口 (mouth) with 荅, which likely contributes the sound. The exact historical development is uncertain.
The mouth 口 is open but silent, and the right side 荅 looks like a person with a blank expression. Together they suggest someone who appears to have forgotten everything.
For トウ, imagine a person with a blank stare, as if their mind is a toe: toe -> トウ, and they've forgotten everything.