The original basis for pretty much all medical treatment was nonsense. Some people noticed some things worked, that got passed down and added to over time, somebody spotted patterns and built up an elaborate theory and then that got passed down as gospel.
The classic example is Hippocrates' theory of the Four Humours. Galen, world-renowned physiologist, elaborated on this by ascribing psychological temperaments to each Humour… To cut a long story short, he taught us that the Central Nervous System controlled the body, and was in turn controlled by the brain (or, at least, the voice was); and that cataracts can be removed with a needle; and that blood provides life-giving energy and nutrition to the body (specifically, heart-generated blood provides energy and liver-generated blood provides nutrition, and blood passes between the chambers of the heart via a blood-permeable interventricular septum); and that men have more teeth than women.
Medicine has moved on since then. If Chiropractors haven't, that's a good argument, but “Chiropractors used to be quacks” isn't a particularly strong argument that they are now.
