ernest
adj.adj. showing that you are very serious and sincere about what you are doing or saying. You use this to describe someone who really means what they say and isn't joking.
adj. characterised by a serious, sincere, and intense state of mind. Often used to describe a person's manner or a specific appeal, implying a lack of irony or playfulness.
Borrowed in the 18th century from Ernst, a medieval royal name in Germany, from Old High German ernust (“vigor, strife”), only remotely related to modern German ernst or English earnest.
Typically precedes the noun it modifies; can also follow linking verbs like 'seem' or 'appear'.
He is very honest about his workHe is very earnest about his workLearners often use 'honest' (truthful) when they mean 'earnest' (serious and sincere in effort).