'overenthusiastic' originates from the prefix 'over-' meaning 'excessively' and the word 'enthusiastic' from Greek 'enthousiasmos', where 'en-' meant 'in' and 'theos' meant 'god'.

'enthousiasmos' transformed into the Latin word 'enthusiasmus', and eventually became the modern English word 'enthusiastic'. The prefix 'over-' was added to denote excessiveness.

Initially, 'enthusiastic' meant 'possessed by a god', but over time it evolved into its current meaning of 'having intense and eager enjoyment or interest'.