I thought that it was a corruption of "Itching Pox" but there are many other explanations. This is from Wikipaedia:
There are many explanations offered for the origin of the name chickenpox:
Samuel Johnson suggested that the disease was "less dangerous", thus a "chicken" version of the pox;
the specks that appear looked as though the skin was pecked by chickens;
the disease was named after chick peas, from a supposed similarity in size of the seed to the lesions;
the term reflects a corruption of the Old English word giccin, which meant itching.
As "pox" also means curse, in medieval times some believed it was a plague brought on to curse children by the use of black magic.
During the medieval era, oatmeal was discovered to soothe the sores, and oatmeal baths are today still commonly given to relieve itching.