What is the origin of the phrase shits and giggles?

I’m pretty sure it’s just a vulgarized version of ‘kicks and giggles’.

Im not sure of the origins but i can tell you that its fairly widely used in australia, and personally i use it quite a bit. Its one of my favorites.
I would guess its just evolved naturally in english from nowhere in particular.

This is the best explanation I could find and actually makes a bit of sense, even though the source is not one I would typically use for reliable information:

Kicks are a measure of amusement, and giggles are a low form of laughter. Hence, the gain is purely trivial or a mood elevator, ... the meaning is greatly skewed to mean: pointless with a hope for gain

John Stewart from “The Daily Show” would know this… since he’s been using it for a couple of years now…

It seems clear to me: doing something that makes you laugh so hard you poop your pants. I can’t hear that expression and not imagine a sound, a stain, and a smell. No, I don’t have evidence of this, but it makes sense to me.