ιχθύς /ichthis(=fish) because in many types of this medical condition there is cracked skin, which is said to resemble the scales on a fish although the scales appear next to each other and the skin is thus more reptilian than fish-like in appearance.
In 1808 Willan described ichtiosis in detail and he stated that “Psoriasia and lepra differ from Ichthiosis in being but partially diffused, and in having deciduous scales”
Ichthyosis is a heterogeneous group of usually congenital skin disorders which are characterised by dry rough and scaly skin due to increased keratinisation of the skin.
Alibert JLM (1806) (in French). Descriptions des maladies de la peau observées a l’Hôpital Saint-Louis, et exposition des meilleures méthodes suivies pour leur traitement. Paris: Barrois l’ainé. p. 286.