I saw this word for the first time today thanks to a link from Stan Carey to a 1922 article. Here it is in the wild:
[Quack is] the favorite epithet for all dissenters from Allopathic faith and practice, and has been applied indiscriminately and with equal ictus to the scholarly founders of the Homeopathic and Naturopathic schools of thought, and the illiterate vendors of hair tonics and corn plasters."
At first, I thought this loquacious writer was just flaunting a large vocabulary (or a new thesaurus). But all of the definitions of ictus seem to equally apply to this writer's sentiment, whether literal or metaphorical. It implies that practitioners of Allopathic medicine use the epithet quack so frequently as to have created a regular rhythm to it, that the word is used as an attack, that it is applied equally (i.e., with equal emphasis) to both studied Homeopaths and illiterate snake oil salesmen, and that it stings. And the fact that the author is using the jargon of Allopathic practitioners against them cannot be overlooked. It's really a masterful word choice.
Too bad it relies an a word that few people have even heard (at least today, 89 years later).