'arachnophobic' originates from Modern English, ultimately from Greek: 'arachne' (ἀράχνη) meaning 'spider' combined with the combining form '-phobic' from Greek 'phobos' meaning 'fear'.

'arachnophobic' developed from the noun 'arachnophobia' (coined in Modern English from Greek elements) by adding the adjectival suffix '-ic' to form the adjective 'arachnophobic'.

Initially formed to denote 'related to arachnophobia (fear of spiders)', and it has retained that specific meaning in modern usage.