This phrase, which means something is clear and beyond dispute or settled in advance, struck me as another hay farming phrase. After mowing/cutting hay, it is left to dry (moisture content is important, so this step takes skill) before it’s ready to bale.
Wiktionary has my guess in mind, saying the phrase comes from herbs being cut and dried for sale, rather than fresh. I suppose that implies the herbs are stable and not going to change – at least, that’s my guess on how the literal and figurative meanings connect.
The first citation of the expression, which must have already been in use or it wouldn’t make sense to use it this way,
is in a letter to a clergyman in 1710 in which the writer commented that a sermon was “ready cut and dried”, meaning it had been prepared in advance, so lacking freshness and spontaneity. worldwidewords
Many people mishear the standard expression as ‘cut and dry.’ Although this form is listed in the Oxford English Dictionary, it is definitely less common in sophisticated writing. The dominant modern usage is “cut and dried.” public.wsu.edu