In a sincere and serious way, showing that you really mean what you say.
From the Old English 'eornest,' meaning 'serious' or 'determined.' The suffix '-ly' makes it an adverb. Medieval Germans also had 'ernst,' showing this seriousness concept spread across Germanic languages.
Earnestness has an interesting reputation—while it means genuine sincerity, it's often mocked as being too serious or overly intense, which is why Oscar Wilde could title a play 'The Importance of Being Earnest' as a clever joke.
Complete word intelligence in one call. Free tier — 50 lookups/day.