Example: If we can pull this one off we'll be in high cotton.
Origin: This came from the old south where cotton was one of the few cash crops when this country was first settled. High cotton is a reference to the tallest healthiest plants, which produced the most cotton. To be "in high cotton" is to have a valuable, bumper crop.
Building on this,
A landowner or worker who had to defecate while in the fields would do so in the field and use some of the cotton for wiping. In the old south, these workers were slaves.
Given a choice anyone would prefer to wipe with the biggest, whitest, fluffiest tufts of cotton which are produced by the tallest, healthiest plants - that is, the high cotton. Since these were also the most valuable plants, one would only wipe with the "high cotton" if they had a very good crop. Otherwise they would seek out less healthy, less valuable samples for wiping.
The complete and original phrase was "sh__ing in high cotton and wiping with the highest, whitest bolls".