Toad-Strangler Meaning

(US, regional, idiomatic) A very heavy downpour of rain.

Example: 1940, Marjorie Kinnan Rawlings, When the whippoorwill:
  "I mean, hit's a toad-strangler of a rain," he proffered. "I mean!" she agreed. "Reckon your daddy'll keer, me a-follerin' this-a-way?" "Reckon he mought."
1943, James Hilton, The story of Dr. Wassell:
  It's not what I'd call a sprinkle, and it's not a chip washer or a gully washer — it's a real regular toad-strangler, and I've seen 'em like this in Arkansas when it turns all the roads into black gumbo.
1998, Sheldon Russell, The Savage Trail:
  It rained in memory of Noah, a toad-strangler that threatened to wash them all into a gully that foamed and churned with red water