taciturn

Pronunciation: \ˈta-sə-ˌtərn\ (listen to it on m-w.com)

Adjective: temperamentally disinclined to speak; silent in expression and manner. (definition at dictionary.com)

Etymology: 18th century; French or Latin. French taciturne (close, taciturn) Latin tacitus (silent, secret).

Usage: Seems to be more often used in descriptions of men, but is not gender specific. Here's a typical usage example of "taciturn" in a snippet from the New Yorker, Volume 79.