Verb: To fall backwards or off of something with an adorable squeal or cry.
The phrase started with the sloth meme featured in the Buzzfeed article titled: "This Baby Sloth Will Remind You To Never Give Up," originally published on September 9, 2015.
When your friend is driving your pregnant wife around, and is such a terrible driver you are worried that your baby might come out pre-shaken, ie sloshen baby syndrome.