'anniv' originates from modern English as an abbreviation of the word 'anniversary', which itself ultimately comes from Latin; in Latin from 'anniversarius', where 'annus' meant 'year' and the element related to 'versus/vertere' meant 'to turn'.

'anniv' is a recent clipped form of modern English 'anniversary'. 'Anniversary' entered English from Old French 'anniversaire', which in turn came from Latin 'anniversarius'.

Originally (from Latin) it meant 'returning yearly' (literally a yearly turning), and over time it came to mean 'a date marking a yearly event' as used in modern English.