This is all I could find:
"The popular name of bear's breeches is thought to be a sanitised version of bear's breech, suggested by the soft, hairy leaves of some species resembling a bear's back legs or rump."
Erin, there is an entire, lengthy article on this subject in the Summer 1996 issue of "Garden History" magazine. Unfortunately, there is only free access to the first page, which does not "spill the beans," as it were. But here it is:
In his book Herbaceous Perennial Plants, (Third Edition) Allan Armitage
talks about this here is a bit of what he says " I felt much better when I learned that the eminent British horticulturist Graham Stuart Thomas was equally stumped. He contacted Professor William Stearn who shed some possible light on the subject. Professor Stearn believed the term resulted from a confusion of the medieval Latin description "branca ursina" meaning bear's claw. The upper part of the hooded flower, upon proper squinting, could be said to resemble a bear's claw. But the breeches, where did they come from? Madame Audrey Serreau, a gardener from Pultiers, France, believes that additional confusion between two Latin words resulted in the "breeches." The word "branca" which means claw, was confused with the old Gaul word "braca" which later became "braies" and finally "breeches."
I have 'Stearn's Dictionary of Plant Names for Gardeners' by William T. Stearn. This book was my companion in collage and helped me to learn my latin ...once I understood the meaning of the word, I never forgot the plants name. Great reference :)