reticent \RET-ih-suhnt\ (adjective) - 1 : Inclined to keep silent; reserved; uncommunicative. 2 : Restrained or reserved in style. 3 : Reluctant; unwilling.
"But poor Harry little knew the difficulty of reticence when the heart is full. He had intended to be very reticent when he came up to London..." -- Anthony Trollope, 'Mr. Scarborough's Family'
Reticent comes from the present participle of Latin reticere, "to keep silent," from re- + tacere, "to be silent."