'annuitant' ultimately derives from the Latin root 'annus' meaning 'year' via the noun 'annuitas/annuity' and the agent-forming suffix '-ant' (from French/Latin).

'annus' (Latin, 'year') produced Late Latin 'annuitas' ('yearly payment'), which passed into Old French as 'annuité' and Middle English as 'annuity'; English formed 'annuitant' from 'annuity' + '-ant' to denote a recipient.

Originally related to 'year' and 'yearly payment' (annuity); over time it came to mean specifically 'a person who receives those regular payments'.