'decay' originates from Middle English/Old French, specifically the Old French verb 'decaer' (or forms like 'decayer'), where the parts come from Late Latin 'decadere'.

'decay' changed from the Late Latin word 'decadere' (de- + 'cadere' meaning 'to fall') into Old French forms such as 'decaer/decayer', then entered Middle English as 'decayen' and eventually became the modern English word 'decay'.

Initially, it meant 'to fall away' or 'to fall down', but over time it evolved into meanings such as 'to rot/undergo decomposition' and more generally 'to decline or decrease'.