earnest

Definitions:

First- fruits; a pledge or assurance of something more to come; earnest- money.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (noun) (n.) (noun)
Seriousness; reality; pledge.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894. (noun) (n.) (noun)
Ardent in the pursuit of an object; eager; urgent; intent; serious; seriously real.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (adjective) (adj.) (adjective)
Showing strong desire: determined: eager to obtain: intent: sincere.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (adjective) (adj.) (adjective)
Determined; eager; intent; sincere.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894. (adjective) (adj.) (adjective)
Money given in token of a bargain made: a pledge: first- fruits: ( fig.) anything which gives assurance, pledge, promise, or indication of what is to follow.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (noun) (n.) (noun)
Money given in hand to assure a bargain; first- fruits; pledge; assurance; in Scot.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (noun) (n.) (noun)
Seriousness: reality.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (noun) (n.) (noun)
Earnestly.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (adverb) (adv.) (adverb)
A portion of something given or done in advance as a pledge.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (noun) (n.) (noun)
Done with a will; with hearty endeavour to attain the end aimed at; eager to obtain; zealous; sincere; serious.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (adjective) (adj.) (adjective)
Serious in speech or action; zealous; eager; ardent; hearty.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (adjective) (adj.) (adjective)
Earnestness.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (noun) (n.) (noun)

Usage examples:

I am quite in earnest and you had better think of it.
- Cousin Henry by Anthony Trollope
An earnest of future felicity!
- Vikram-and-the-Vampire-Classic-Hindu-Tales-of-Adventure-Magic-and-Romance by Burton, Isabel, Lady
Perhaps she was in earnest after all.
- Don Orsino by F. Marion Crawford