'astrogeologist' is formed from the combining form 'astro-' and the noun 'geologist'. 'astro-' originates from Greek, specifically the word 'astron', where 'astron' meant 'star'. 'geologist' comes from Greek elements 'geo-' (earth) and 'logos' (study/speech) via New Latin and Modern English.

'astro-' (from Greek 'astron') was attached to existing English scientific nouns; 'geologist' derives from New Latin/Greek 'geōlogos' and entered English in the 19th century. The compound 'astrogeologist' arose in modern scientific English (20th century) as planetary science developed, combining the two elements into the current term.

Individually, 'astron' meant 'star' and 'geologist' meant 'one who studies the earth'; combined, the compound evolved to mean 'one who studies the geology of celestial bodies (not limited to Earth)'.