'arachnophagous' originates from Modern Latin/Neo-Latin, specifically the combining form 'arachno-' from Greek 'aráchnē' meaning 'spider' and the suffix '-phagous' from Greek 'phāgós' meaning 'eating'.

'arachnophagous' was formed in English from Neo-Latin 'arachnophagous' (a compound of Greek elements) and entered scientific/technical usage in the 19th–20th centuries as a descriptive term.

Initially, the components meant 'spider' and 'eating', and the compound has retained that core meaning as 'feeding on spiders'.