Toad (n.)
c. 1300, from late Old English tadige, tadie, of unknown origin and according to OED with no known cognates outside English. Applied to loathsome persons from 1560s. Toad-strangler "heavy rain" is from 1919, U.S. Southern dialectal.

Source : Online Etymology Dictionary

Note: The Online Etymology Dictionary maintains a free-access list of the origins of thousands of English words, a significant proportion of which are unknown