'unavailability' originates from the prefix 'un-' meaning 'not' and the word 'availability', which comes from 'available', derived from the Old French 'avaler', meaning 'to descend'.

'availability' changed from the Old French word 'avaler' and eventually became the modern English word 'available', with 'unavailability' forming by adding the prefix 'un-'.

Initially, 'available' meant 'to be at hand or ready for use', and 'unavailability' evolved to mean the opposite, 'not being at hand or ready for use'.