sincere · adjective /sɪnˈsɪr/
sincere
C1 honest and saying or showing what you really feel or believe
Dictionary example:
He seems to be sincere.
Learner example:
During these years I have also found her to be honest, sincere, hard-working and imagin[a]tive. (Certificate in Advanced English; C1; Swedish)
sincere apologies/thanks, etc.
C1 used to add emphasis when you are expressing a feeling
Dictionary example:
The family wishes to express their sincere thanks to all the staff at the hospital.
Learner example:
Again, sincere gratitude for your kind work. (Certificate in Advanced English; C1; Chinese)