Duckin or duck is untweilled linen, later untwilled cotton, a shade finer than canvas. OED says it was used on smaller sails and on sailor's outer garments. OED cites usage from 1640 on and says the trou were much in use c.1800-50.

It's originally a ref. to the material, which could be used for anything, possibly trousers but also smocks. The case would be exactly parallel to "blue denims" by which I think almost everybody would understand trousers/"jeans" though there are denim jackets in this world.

I first heard the word from a fellow history re-eactor who "is" a c.1800 privateer & as such wears white duckin trousers. In the "it's a small world after all" dept., I just within the last week bought Carl Sandburg's version of "Boll Weevil" on a CD from Amazon.com), and heard that word for the second time. I gather that cotton-picking is rough on the hands, & evidently a picker might want clothing that wasn't going to tear easily from snagging on the plants.

Origin is "doeck," Dutch for "linen."

CC