Sincerity

Sincerity

freedom from deceit, hypocrisy, or duplicity; probity in intention or in communicating; earnestness.
Sincerity is the virtue of one who speaks and acts truly about his or her own feelings, thoughts, and desires.

"Truthfulness or sincerity is a desirable mean state between the deficiency of irony or self-deprecation and the excess of boastfulness." (Aristotle)

Etymology
The Oxford English Dictionary and most scholars state that sincerity from sincere is derived from the Latin sincerus meaning clean, pure, sound (1525–35). Sincerus may have once meant "one growth" (not mixed), from sin- (one) and crescere (to grow). Crescere is cognate with "Ceres," the goddess of grain, as in "cereal."
According to the American Heritage Dictionary, the Latin word sincerus is derived from the Indo-European root *sm̥kēros, itself derived from the zero-grade of *sem (one) and the suffixed, lengthened e-grade of *ker (grow), generating the underlying meaning of one growth, hence pure, clean.