'nonannual' is a Modern English compound formed from the prefix 'non-' (meaning 'not') and 'annual' from Latin 'annuus', ultimately from Latin 'annus' meaning 'year'.

'annual' came into English via Old French 'annuel' from Latin 'annuus' (from 'annus' meaning 'year'); the productive English prefix 'non-' (from Latin 'non') was attached in Modern English to form 'nonannual'.

Originally 'annual' meant 'pertaining to a year'; 'nonannual' was formed to mean 'not yearly' and has retained that negative/contrastive sense.