stutter
訥 centers on speech that does not flow smoothly: stuttering or speaking haltingly.
訥 combines 言 (speech) with 内, which likely contributes the sound and may suggest something held inside, relating to speech that is blocked or hesitant.
言 (speech) beside 内 (inside) suggests words stuck inside, unable to come out smoothly — a stutter.
For トツ, imagine a person stuttering on the word 'toast': 't-t-toast' — the repeated 't' sound cues トツ.