laugh, chew, eat
咥 centers on actions involving the mouth: laughing, chewing, and eating. The laugh sense is a vocal expression, while chew and eat are physical mouth actions.
咥 combines 口 (mouth) with 至, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound where 口 indicates the meaning relates to the mouth.
The mouth 口 is open wide, and the right side 至 looks like a person arriving with food. Picture someone laughing as they arrive to chew and eat.
For テツ, imagine a person laughing so hard they can't chew their food, and the sound they make is a sharp 'tets' as they try to eat: tets -> テツ.
to hold in one's mouth