A cantharidal blister caused by the vesicating body fluid of certain beetles.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
Examples from the Web for fly blister
Historical Examples of fly blister
She went to a doctor for the first time in her life and was given a fly-blister and some drugs to put in whiskey.The Life and Work of Susan B. Anthony (Volume 1 of 2)
Ida Husted Harper