taciturnity (n.)

c. 1600, "silent, unspoken," from French tacite and directly from Latin tacitus "that is passed over in silence, done without words, assumed as a matter of course, silent," past participle of tacere "be silent, not speak," from suffixed form of PIE root *tak- "to be silent" (source also of Gothic þahan, Old Norse þegja "to be silent," Old Norse þagna "to grow dumb," Old Saxon thagian, Old High German dagen "to be silent"). The musical instruction tacet is the 3rd person present singular of the Latin verb. Related: Tacitly.

"habitually silent," 1771, back-formation from taciturnity, or from French taciturne (15c.), from Latin taciturnus "not talkative, noiseless."