rhesus, n. (in full rhesus monkey) a small catarrhine monkey, Macaca mulatta, common in N. India.
rhesus baby an infant with a haemolytic disorder caused by the incompatibility of its own rhesus-positive blood with its mother's rhesus-negative blood. rhesus factor an antigen occurring on the red blood cells of most humans and some other primates (as in the rhesus monkey, in which it was first observed). rhesus negative lacking the rhesus factor. rhesus positive having the rhesus factor.
mod.L, arbitrary use of L Rhesus f. Gk Rhesos, mythical King of Thrace