'indexer' is formed in Modern English by adding the agentive suffix '-er' to 'index' (from Latin 'index').

'index' comes from Latin 'index' (meaning 'sign, pointer, list'), which entered English via Middle French and Middle English; the agentive English suffix '-er' (Old English/West Germanic) was later attached to create 'indexer'.

Originally 'index' in Latin meant 'a sign or pointer'; over time in English it came to mean 'a list of topics or names with pointers to their locations', and 'indexer' came to mean 'one who makes such a list' and later also 'a program that creates such a list for electronic search'.