Reticent
Etymology
From Latin "reticere" (to keep silent), combining "re-" (an intensifier) with "tacere" (to be silent) — the same root as "tacit." The word entered English in the nineteenth century meaning specifically disinclined to speak.
adjective · /RET-i-suhnt/
Reluctant to speak or to reveal one’s thoughts and feelings.
What it names
It captures a specific reserve — holding back what one could say — rather than general shyness or a lack of things to say.
Example
He was warm about most subjects but reticent whenever the conversation turned to his family.
Related idea
Increasingly, and loosely, used to mean simply “hesitant” or “reluctant” to do anything — a drift purists resist. Distinguish from taciturn, which describes a habitually silent temperament; reticence is about withholding particular things, taciturnity about a general sparseness of speech.