From Kimball Attwood at Science Based Medicine (Homeopathy & Evidence-Based Medicine)
“Allopathy” was the term coined by Hahnemann to dramatize the contrast between homeopathy and its competition. According to Hahnemann, “allopathic” medicine sought to give only medicines that suppressed symptoms:
Frequent Science-Based Medicine commenter Jan Willem Nienhuys (I don't remember if he's an MD or not)
A small aside: Weil refers to “allopathic medicine”. The term “allopathic” is pathognomonic for brain rot. Originally it was coined by Samuel Hahnemann to indicate the evils of 18th century medicine, as opposed to his homeopathy. The etymology for “allopathy” was medicine that does always something inappropriate. More specifically: treatment with bloodletting, purgatives, emetics, klysmas, cupping, artificial ulcers, scarification, acupuncture, moxibustion, cauterisation and etching, and “drugs” consisting of crazy herbal mixtures and/or metallic compounds (such as lead or copper oxide), in general, “dangerous, violent medicines”. In short, what we nowadays call naturopathy.
The chiroquacktors use the word allopathy a lot to disparage medicine (as opposed to their fantasy world).