bosotto
Speaking in a low, barely audible mumble — often a complaint or comment you don't really want anyone to hear. The words just slip out.
💡After being scolded — 'He said bosotto, "That's not fair..."'
彼はぼそっと「ずるい」とつぶやいた。
He muttered bosotto, 'That's not fair.'
She said ___, 'I didn't want to come anyway.'
「別に来たくなかったし」と○○つぶやいた。