'annualise' originates from Latin, specifically the word 'annus', where 'annus' meant 'year'. The verb form in English is created by combining 'annual' (from Latin 'annualis') with the verb-forming suffix '-ise' (from French '-iser' or English '-ize').

'annual' developed from Latin 'annualis' (from 'annus') into Middle English as 'annual', and later the verb was formed in modern English as 'annualise' (and the US spelling 'annualize') to mean 'make yearly' or 'express on a yearly basis'.

Initially it related to being 'of or pertaining to a year' ('yearly'), but over time it evolved into a verb meaning 'to convert or express on a yearly basis' and an adjective meaning 'converted to an annual rate'.