from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A small flatbottomed boat with square ends, paddled or poled on shallow waterways.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. a flat bottomed boat with a very shallow draft, whose bow and stern are both squared off
Probably from the name John.(American Heritage® Dictionary of the English Language, Fourth Edition)