toad: [OE] Toad is a mystery word, with no known relatives in any other Indo-European language. Of its derivatives, toady [19] is short for the earlier toad eater ‘sycophant’ [17]. This originated in the dubious selling methods of itinerant quack doctors. They employed an assistant who pretended to eat a toad (toads were thought to be poisonous), so that the quack could appear to effect a miraculous cure with his medicine.
The toad-eating assistant came to be a byword for ‘servility’ or ‘dependency’, and hence for ‘servile flattery’. Toadstools [14] were named for their stool-like shape, and also because of an association between poisonous fungi and the supposedly poisonous toad.
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.