'post-noon' is formed from the Latin prefix 'post-' meaning 'after' and the word 'noon' (from Old English 'nōn', from Latin 'nona hora' meaning 'ninth hour').

'post-' entered English as a productive prefix from Latin 'post' in Late Latin/Medieval use; 'noon' developed from Old English 'nōn' (through Middle English 'none'/'noon') which in turn came from Latin 'nona hora'. The compound 'post-noon' is a later, literal formation in English to mean 'after noon'.

Initially, 'noon' referred to the 'ninth hour' (roughly mid-afternoon in ecclesiastical reckoning), but over time it shifted to mean 'midday' (around 12:00). 'Post-noon' therefore came to mean 'after midday' or 'afternoon' in modern usage.