From Old English (before 12th century), from Proto-Indo-European '*h₃es-' ("summer, harvest"), from PIE *h₃es- ("summer, harvest").
To receive money in return for labor or services; to gain deservedly as a result of one's actions.
From Old English 'earnian' (to earn, deserve, merit, labor for), from Proto-Germanic '*aznojan' or '*azanōną' (to earn, to harvest), possibly related to Proto-Germanic '*asanaz' (harvest time, summer), from PIE root *h₃es- (summer, harvest). The original connection was agricultural — you 'earned' by harvesting, by reaping what you had sown. The same root produced German
'Earn' is etymologically connected to 'harvest' — both trace back to a Proto-Germanic root meaning 'reaping time.' The German word 'Herbst' (autumn) is from the same family, because autumn was when you literally earned your living by bringing in the crops. Every paycheck is, etymologically, a harvest.