To "ret" is to soak plant material (flax, jute, hemp, kenaf, etc) to loosen the fibers.
Wikipedia notes that you can also ret by leaving the crop in the fields, and that that process is called "dew retting." Hence the name of the famous cartoon botanist Dudley Dew-Ret, and that Beatles song about hemp fiber, "Ticket to Ret," featuring the line "She ought to think twice, she ought to dew ret by me."
In related news, Jed is kind of punchy at the moment. In case that wasn't obvious.