Jason Smith was showing his mechanism for the first functional Pentultimate (by Jason Smith). at a Berkley twisty puzzle meetup when he was told flatly by Bram Cohen, "Only a true knucklehead would actually try that."
Jason agreed, and later adopted the term himself to describe the mechanism as the "Knucklehead Mechanism" in the thread announcing the first 24-Cube. 
It is a term used not with scorn, but great affection and wonder at the persistence and insanity to go where others would never tread for sound and sensible reasons.
A mechanism others had roundly discarded as completely impractical, created only with great time, expense and effort.
Pursuit or completion of a series generally thought to be far too difficult, expensive or time consuming to ever be practical.