From Old English *eornost* ("seriousness, zeal"), related to Old High German *ernust* ("earnestness, combat"). The word has carried a sense of deep sincerity since at least the 10th century, and by the 16th century it was firmly established as both an adjective and a noun (as in "in earnest," meaning seriously or with full intent).

The phrase "in earnest" is extremely common in English and means "seriously" or "with real effort" — e.g., *Work began in earnest after the holidays.* Be careful not to confuse *earnest* with *ernest* (a personal name, as in Oscar Wilde's play *The Importance of Being Earnest*, whose title cleverly plays on both senses).