Sincerity represents a strict correspondence between an individual’s professions and their actual feelings and beliefs. Sincere communication is thoroughly honest — free of hypocrisy, pretense, deceit, and contradictions.
Insincerity can be evinced when we act as if our feelings for something are stronger than they are: when we compliment someone out of a desire for advantage rather than genuine admiration; smile affirmingly while inwardly rolling our eyes; accent our messages with a number of exclamation marks which belie our true enthusiasm.
Yet a form of insincerity that is arguably even more common, and still harder to overcome, is that of pretending our feelings are less strong than they actually are: when we leaven the sharing of deeply-held dreams with rueful deprecation; disguise the heartfelt intent behind a gift by presenting it with jokey patter; cloak a genuine opinion in sarcasm; follow a confession of delight with an ironic wink and nod.
‒ https://www.artofmanliness.com/character/advice/sunday-firesides-yours-sincerely/