つぶやtsubuyaku
1.

To speak softly or quietly, often expressing thoughts or feelings aloud in a way that may not be intended for others to hear, typically resembling a soliloquy or a personal reflection.

2.

To express dissatisfaction or complaints in a subdued tone, often indicating a lack of enthusiasm or a sense of resignation.

3.

To utter words in a low voice as if to oneself, often conveying a sense of contemplation or introspection.

Example:

その内気な少年はぼそぼそと自分の名前をつぶやいた。