A useful, simple, pragmatic definition of sincerity:
“Saying on the outside what’s happening on the inside”
… so that when you ask, promise, declare or inquire those who are listening are offered the possibility of trusting what you have to say
… so that when you speak you are also offered the possibility of trusting what you have to say
… and so we don’t have to spend so much of our time and energy playing games – trying to figure out what each other want, and need, and are really committed to.