WAG but, Lissa, could the pudding usage of dick be the provenance of Grandma’s expression?
Phrase etymology:
http://www.libraryspot.com/dictionaries/etymologydictionaries.htm
http://www.fun-with-words.com/etym_phrases.html
GOOGLE “phrase etymology” for more sites.
My personal favorite is “Face like a pitbull licking piss off a thistle”.
Yep, see it Here folks.
“I’ll whup you upside the head, I will.”
…thus speaketh a 'Merkin. We knows how to speak proper English, we does.
You should’ve named this thread, “Confusing Yanks For Fun And Profit.”
Just as a precaution, I’m not the stereotypical American gun-nut bible-thumping hillbilly imbecile from your fancy british TV shows. I hate rednecks with a passion.
Adding to the mix: What are some af the worst derogatory slangs for “American” that you guys know? Gimme your best shot. 
…while chewing a wasp
…and Bob’s yer uncle
Or better yet: "I’ll whup you upside yo head with a 2 X 4 'bin soaked for 6 months in a barrel of used motor oil, heah?