Sincere means you actually mean it — no act, nothing put on for the room. A thank-you that sounds sincere because the person names the thing you did; an apology that sounds sincere because it does not come with an excuse attached.
The noun is sincerity and the adverb sincerely, and 'Yours sincerely' still closes a British letter to someone whose name you know. Sincere attaches to apologies, thanks, regret, condolences, admiration, belief and effort — words for what a person feels rather than what they do. The opposite, insincere, is the sharper word of the two, because it accuses you of performing. Watch the limit: sincere is about honesty, not accuracy, so a person can be sincerely and completely wrong. It also runs a notch above the everyday — an ordinary thank-you is just a thank-you, and calling it sincere means you are vouching for it.
1adjectivesincereMeaning what you say or feel, with nothing put on for effect.
“Her praise sounded sincere, and he found himself believing it.”
Also accepted in games: genuine
The famous story about this word is false. Sincere does not come from Latin sine cera, 'without wax', with statues sold uncracked and unfilled; that is a folk etymology. Latin sincerus meant genuine, and behind it sit the roots *sem-, 'together', and *ḱer-, 'grow' — the growing root that also gave Ceres, goddess of the harvest, and the word cereal. Something sincere grew whole, of one piece.
Middle French
sincere
English, today
sincere
Built from
Borrowed from Middle French sincere. From Middle French sincere, from Latin sincerus (“genuine”), from Proto-Indo-European sem- (“together”) (whence sam#Adverb) + ḱer- (“grow”) (whence Ceres, the goddess of harvest, etymon of cereal). Not from sine (“without”) + cera (“wax”), which is a folk etymology; see Wikipedia page.
Etymology from Wiktionary, CC BY-SA