呟く
つぶやく · tsubuyaku · JLPT N1 · verb-godan
to mutter
つぶ (tsubu, a small bead or grain) + やく: murmur out small, quiet bits of speech under your breath
つぶ means a tiny grain or drop, and 呟く is a quiet, half-formed word spoken under your breath. Think of words coming out as small crumbs, not a full sentence.