The story
From least + weasel, from the fact that it is the smallest member of its genus.
Etymology from Wiktionary, CC BY-SA 4.0.
least weasel
least wea·selToday it means: The common weasel, little weasel, or simply weasel (Mustela nivalis), the smallest member of the weasel genus Mustela, native to Eurasia, North America, and North Africa. Full definition →