'overzealous' originates from the prefix 'over-' meaning 'excessive' and the word 'zealous' from Medieval Latin 'zelosus', where 'zelosus' meant 'full of zeal'.

'zealous' changed from the Medieval Latin word 'zelosus' and eventually became the modern English word 'zealous', with 'overzealous' forming by adding the prefix 'over-'.

Initially, 'zealous' meant 'full of zeal', but 'overzealous' evolved to mean 'excessively enthusiastic'.