This is used to describe people who exclusively use Sitka branded gear, and think they are better than everyone else because of it. Sitka's motto, "sick for it" is a common hashtag people use on social media when posting about Sitka gear, hence the disease "Sick-for-it-itis" . People that suffer from this disease are usually cocky, and look down upon others because they spend thousands of dollars on mediocre, at best, gear.
"We knew Bobby had a strong case of Sick-for-it-itis when his entire instagram account consisted solely of Sitka gear pictures and #sickforit hashtags."
by DeekDog97 December 26, 2016