from The Century Dictionary.
- noun One who navigated a flat-boat: used especially in early river navigation in the Ohio and Mississippi valleys.
Sorry, no etymologies found.
I doubt the rascal did not come by it very honestly, neither was a costly bracelet that one of my father's best hands (once a Northwich salt-flatman) brought home for my baby sister.
Page 269 look after the trunks or locks which shut out the water from the ditches), a cow and sheep-minder, an ostler, a flatman, and a boatman.