Being sincere is being genuine. If someone compliments you on your outfit, but you notice them rolling their eyes when they think you’re not looking, they probably weren't being sincere.

If you've been offered a sincere thanks for something you've done, the person is probably really grateful. Sometimes when people say "thank you," they don't always mean it, but when they add the adjective sincere, the emphasis is usually for a very good reason. You may hear a charity sending out a sincere thanks to sponsors for their donations, which help keep the charity running.

Definitions of sincere

adj open and genuine; not deceitful

“he was a good man, decent and sincere
“felt sincere regret that they were leaving”
sincere friendship”
echt, genuine
not fake or counterfeit
honest, honorable
not disposed to cheat or defraud; not deceptive or fraudulent
artless, ingenuous
characterized by an inability to mask your feelings; not devious
existent, real
being or occurring in fact or actuality; having verified existence; not illusory
bona fide
undertaken in good faith
sincerely or intensely felt
dear, devout, earnest, heartfelt
without dissimulation; frank
genuine, true, unfeigned
not pretended; sincerely felt or expressed
heart-whole, whole-souled, wholehearted
with unconditional and enthusiastic devotion
consistent with fact or reality; not false
lacking sincerity
counterfeit, imitative
not genuine; imitating something superior
dishonest, dishonorable
deceptive or fraudulent; disposed to cheat or defraud or deceive
artful, disingenuous
not straightforward or candid; giving a false appearance of frankness
lacking in reality or substance or genuineness; not corresponding to acknowledged facts or criteria
bootlicking, fawning, obsequious, sycophantic, toadyish
attempting to win favor from influential people by flattery
buttery, fulsome, oily, oleaginous, smarmy, soapy, unctuous
unpleasantly and excessively suave or ingratiating in manner or speech
concealing under a false appearance with the intent to deceive
deliberately deceptive
not genuine
gilded, meretricious, specious
based on pretense; deceptively pleasing
professing feelings or virtues one does not have
given to or characterized by presenting specious arguments
not in accordance with the fact or reality or actuality
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adj characterized by a firm and humorless belief in the validity of your opinions

“an entirely sincere and cruel tyrant”
earnest, solemn
concerned with work or important matters rather than play or trivialities

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