Learner's definition of SINCERELY
genuinely or truly
in a sincere or truthful way
He sincerely apologized for breaking the vase.
Do you sincerely [=really] believe that the country is better off now?
I sincerely hope that you do a better job next time.
I am sincerely sorry for your loss.
He tried sincerely to make up for his mistake.
◊ Sincerely, yours sincerely, and (chiefly US) sincerely yours are often used at the end of a formal letter before the sender's signature.