We thought he was joking at first, but then we realized that he was 'in earnest'. [=he was being serious]
It's hard to tell if he's making this proposal 'in earnest'.
It began to rain 'in earnest' [='heavily'] in the evening.
(..) to betray's in deepest consequence. MACBETH: This supernatural soliciting cannot be ill, cannot be good. If ill, why hath it given me earnest of success, commencing in a truth? I am Thane of Cawdor. If good, why do I yield to that suggestion whose horrid image doth unfix my hair (..)2015 Macbeth
(..) in his kingdom's great defence, and poured them down before him. And we are sent to give thee from our royal master thanks. And, for an earnest of a greater honour, he bade me from him call thee Thane of Cawdor. The Thane of Cawdor lives. Why do you dress me in borrowed robes? (..)2015 Macbeth