It's jargon.

A choocher is a thing that chooches; it's a machine that accomplishes things. Like a steam locomotive, chooching along and doing real work as it goes.

Skookum is a term borrowed from Chinook that means things like strong, brave, or excellent. When an object is skookum, it is resoundingly well-made, robust, or reliable. Some things are more skookum than others, while some aren't skookum at all.

A skookum choocher is a well-made machine that does a great job of doing stuff.