'astrologist' originates from Medieval Latin/Latinized forms such as 'astrologus', ultimately from Greek 'astrologos', where 'astron' meant 'star' and 'logos' meant 'word/reckoning/study'.

'astrologos' (Greek) changed into Latin/Medieval Latin 'astrologus', and through Old/Middle English influences entered modern English as 'astrologer' and the related formation 'astrologist'.

Initially it referred simply to someone who studied or spoke about the stars; over time it came to mean a practitioner who interprets celestial positions to predict or explain human affairs (modern sense of 'practitioner of astrology').