'over-explain' originates from the prefix 'over-' meaning 'excessively' and the verb 'explain' from Latin 'explanare', where 'ex-' meant 'out' and 'planare' meant 'to make plain'.

'explanare' transformed into the Old French word 'expliquer', and eventually became the modern English word 'explain'. The prefix 'over-' was added to form 'over-explain'.

Initially, 'explain' meant 'to make plain or clear', and with the addition of 'over-', it evolved to mean 'to explain excessively'.