“one who,” “specialist in” — Agent-forming suffix — “person who practices or specializes in”
The suffix entered English in the 1640s as physiologist (noun), borrowed from French physiologiste, from Late Latin physiologus, from Greek φυσιολόγος (physiologos) — literally “one who discourses on nature” (physis “nature” + logos “word, study”). The term toxicologist appeared in the 1840s, and pathologist in the 1790s. The root logos + -istēs (“one who studies”) connects -logist to the entire specialist and practitioner suffixes: cardiologist (cardio- + logist → one who studies the heart), neurologist (neuro- + logist → one who studies the nervous system), and pharmacologist (pharmaco- + logist → one who studies drugs). The agent-forming compound suffix-logist is one of the most productive suffixes in medical terminology: e.g., toxicologist, radiologist, hematologist, nephrologist, and dermatologist.