earnest (2) — ODE

earnest2 /ˈəːnɪst

▸ noun [in singular] a thing intended or regarded as a sign or promise of what is to come:

the very deliberateness of their disguise is an earnest of their real aloofness.

– ORIGIN Middle English ernes, literally ‘instalment paid to confirm a contract’, based on Old French erres, from Latin arra, shortened form of arrabo ‘a pledge’. The spelling was influenced by words ending in -ness; the final -t is probably by association with earnest1.