taciturn

habitually silent

Root

tac: silent

Other words with the tac root

reticence: reluctance to speak
reticent: restrained or reluctant to speak
tacit: understood without being spoken

Tac means silent. Taciturn is silence as a habit: few words, even when others expect more.

The taciturn coach said little, so every sentence carried weight.

Reticent can be situational: someone holds back today. Taciturn feels more like temperament. Silence is the default setting, not just a temporary choice.