stutter
吶 centers on disrupted speech: stuttering or speaking haltingly. The meaning is narrow and directly tied to the act of stammering.
吶 combines 口 (mouth) with 內, which likely contributes the sound and may suggest speech held inside, fitting the stutter meaning. The exact historical development is uncertain.
The mouth 口 beside 內 (inside) suggests words stuck inside the mouth, unable to come out smoothly—like a stutter.
For トツ, imagine a person trying to speak but only managing a sharp, halting 'tots' sound: tots -> トツ, capturing the abrupt stop of a stutter.