'over-sensitively' is formed in modern English from the prefix 'over-' and the adjective 'sensitive'. 'over-' originates from Old English, specifically the word 'ofer', where 'ofer' meant 'above, beyond, excessively'; 'sensitive' ultimately originates from Latin, specifically the word 'sensitivus', where the root 'sentire' meant 'to feel'.

'sensitive' changed from Latin 'sensitivus' into Old French (e.g. 'sensitif') and then into Middle English as 'sensitive'; the productive English prefix 'over-' comes from Old English 'ofer' and has long been used to form adjectives meaning 'excessively X', producing forms like 'over-sensitive' and then the adverb 'over-sensitively'.

Originally, elements meant 'able to feel' (from Latin 'sentire' → 'sensitive' = 'capable of perception/feeling'), but the compound with 'over-' came to mean 'excessively or unduly sensitive' and the adverb now means 'in an excessively sensitive manner'.