Where did "the worms crawl in, the worms crawl out, crawl in your tummy and out your mout' "come from?

Are you sure you would like the answer to this Cosmic?? 
O.K..... Here are some thoughts as to the origin:

* Any grave robber

* Quincy
* An upgrade from Quincy,  Bones
* Of course, Scully from the X-Files.  She actually reveals much more that crawl in and out of your bones 

*The unforgettable "Hearse Song" This post was edited by Merlin at July 29, 2018 12:48 PM MDT

Probably another plague song. 

Just an old folk song....has been around since 1800s...no one really knows the origin.

In the beginning, theater stages did not have curtains, so actors had to be off the stage somehow at the end of each act. If a character died, they certainly didn't want the corpse getting up and walking off, so the playrights invented the funeral procession to carry the body away.

"The Hearse Song" as it is called by historians was first documented in the 19th century, but many people think it dates from WW1. And that's as good as the story gets.

BTW Alfred Hitchcock's theme is a funeral march from opera.

have no idea

I don't think I wanna know. Ew!