The folk etymology suggests that there used to be an island in Drake’s Creek where slaves were kept shackled or where there was a shack where people used to drink even when it was illegal.
The problem with both of these theories is that Shackle Island was called that almost as soon as white people got to Sumner County, back before there was any real slave trading or before it was illegal to drink.
I have another theory.
A shackle in old-timey boat parlance was a length of cable fifteen fathoms long–about thirty yards.
Shackle Island is visible on the LOC map. You can see it up Drake’s Creek where Long Hollow Pike crosses it. It is conveniently labeled. Though the island no longer exists, I did my best to roughly sketch in the eastern boundary of the island.
The distance from the eastern side of the island to the western side of the island would have been almost 900 feet.
Edited to add: Holy shit! Is there anything my bad math won’t ruin. Thirty yards is 90 feet, not 900.