'unserious' originates from the prefix 'un-' meaning 'not' and the word 'serious', which comes from Latin 'serius', meaning 'weighty or important'.

'serius' transformed into the Old French word 'serieux', and eventually became the modern English word 'serious'.

Initially, 'serious' meant 'weighty or important', but 'unserious' evolved to mean 'lacking seriousness'.