Singing is the poop hole loop hole to all topics you aren't allowed to talk about.

doesn't seem to be a clear answer. Wikipedia says it comes from oltre-eschelture an old French form analogous to "ultra-kilter," i.e. outside of an orderly state. other sources say it's a reduplication (like "super-duper" or "fancy-schmancy") based on the Germanic word skelt, which Wiktionary claims means to yell or be noisy, related to "scold," while Webster claims that skelt means to come and go