'undesirable' originates from the prefix 'un-' meaning 'not' and the word 'desirable' from Latin 'desiderabilis', where 'desiderare' meant 'to long for'.

'desiderabilis' transformed into the Old French word 'desirable', and eventually became the modern English word 'desirable', with 'un-' added to form 'undesirable'.

Initially, it meant 'not to be desired', and this meaning has largely remained the same in modern usage.