One of the things I love about our home is that my wife keeps an unabridged dictionary open on a stand in our living room. Whenever she looks up a word which she comes upon in her voracious reading, she makes a little pencil tic next to the entry. I have no such habit, but I often find myself stopping and perusing whatever page it happens to be open to. The other day my eye caught the entry for earnest, and I was brought up short by what I saw.
Now, I have frequently heard the expression earnest-money used in (boring grown-up) conversation, and to my mind it made perfect sense: a sum of money paid to assure one party that the other is serious, earnest in their intention. As I said, it makes sense, and I would never given the concept any further thought. But there are in fact two completely independent words at work here, which evolved into an identical spelling in modern English.
The earnest we think of (now almost exclusively used adjectivally) is taken from Old English eornust and goes back to an unknown base word which also provided the source for the Old Norse ern meaning ‘brisk, vigourous’ and the Gothic arniba ‘safely’ (New Shorter Oxford English Dictionary). As a noun, it means “seriousness, as opposed to jest” (op. cit.).
The other earnest comes to us from a Middle English word — arles —representing a mediæval diminutive of the Latin word arrha, itself derived from the Greek arrabōn, all meaning ‘money given as a pledge or to bind a transaction’ i.e. earnest-money. An earnest can also be used figuratively to mean any pledge or foretaste of a person’s intentions. The final form of the second was deliberately aligned with the form of the first somewhere along the way, thus giving us our two words in one.
And now we know that.