reticent

reticent \RET-ih-suhnt\, adjective:

1. Inclined to keep silent; reserved; uncommunicative.
2. Restrained or reserved in style.
3. Reluctant; unwilling.

His wispy eyebrows sit above eyes undimmed by more than forty years of serious scholarship; a tight-lipped smile suggests that there are many things he will not say about himself or his accomplishments. Indeed, he is almost painfully reticent about what most scholars now consider to be a monumental achievement in the field.
-- Marc K. Stengel, "The Diffusionists Have Landed", The Atlantic, January 2000

Within a circle of intimate friends, he's a very sociable person, says Russell Banks, another novelist, who has known Auster since 1977. "Outside of that circle, he's fairly shy and reticent."
-- "Case of the Brooklyn Symbolist", New York Times, August 30, 1992

People might be reticent to put a more sizable amount into their 401(k) because they're worried it will affect their lifestyle.
-- Alexandra Zendrian, "Feel The Retirement Burn", Forbes, July 29, 2009

Reticent comes from the present participle of Latin reticere, "to keep silent," from re- + tacere, "to be silent."