'overreactivity' originates from English, formed by the prefix 'over-' (from Old English 'ofer') meaning 'beyond, excessive', combined with 'react' (from Latin roots 're-' + 'agere' via modern English meaning 'to act in response') and the noun-forming suffix '-ity' (from Latin '-itas').

'overreactivity' developed in modern English by adding the suffix '-ivity' (from 'reactivity') to the verb 'overreact'; 'overreact' itself is a combination of the prefix 'over-' + 'react.' The compound arose through productive English word-formation rather than from a single older borrowing.

Initially related broadly to 'the quality of reacting,' the compound has come to denote specifically an excessive or abnormally strong reaction (emotional or physiological).