noun
sincerity
sin-SAIR-uh-tee/sɪnˈserəti/
Honesty in expressing what one truly feels or believes.
The stranger's sincerity shows in the heartfelt thanks for the returned wallet.

Etymology
The noun ending makes sincerity the quality of being pure or genuine. Latin sinceritas meant purity or wholeness, an image carried into honest feeling and expression as freedom from deceit; the base’s deeper origin is uncertain.
In context
Flattery, when used with sincerity and discretion, helps you gain trust and influence without coming across as manipulative.