A person who acts as or serves in the role of an aunt.
From 'aunt' (from Old French 'ante', from Latin 'amita' meaning father's sister) plus the agent suffix '-er', which creates a noun for someone who performs an action or holds a role.
The suffix '-er' is one of English's hardest workers—it transforms almost any verb into a person who does that thing, but here it's being applied to a kinship role, showing how English speakers naturally extend this suffix to any role they want to nominalize.
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