A side road used to avoid the tolls or traffic of a turnpike or highway.
Etymology: shun + turnpike. mid-1800s
"The Doctor overwhelms us with learning, universal and incomprehensible. Here is his map of the mental faculties, in which twenty-eight separate powers of mind are described in odd and even numbers.
"The arrows show the course of ideas through the mind. They begin with the motives in the region of the highest odd numbers in the south-west corner of the mind, marked A, and go perpendicularly northward, through Thirst and Hunger to Sensation, marked B; then turn to the right and go eastward, through Conception, to Attention, marked C, and then descend southward, through Perception, Memory, Understanding, Comparison, Combination, Reason, Invention, and Judgement; wheel to the left under Will, marked D, and pass through Conscience, and then to V, the unascertained centre of Sensation, Volition, and Will. This is the natural turnpike road for ideas when we are awake and sane. But here is an open shunpike, X, Y, Z, on which Ideas, when we are asleep or insane, start off and pass by Conscience, and so avoid paying toll to that inflexible gatekeeper."
- William H. Seward, in his defense of William Freeman, 1846.
(you can see the whole strange diagram in this book, on page 356,
and here is a little more about William Freeman)