Things that are no longer of any usefulness are obsolete; things of declining usefulness are obsolescent. I feel a bit of the latter as I approach 70 years of age. Then comes the former.
The spelling lacks the letter b, although the word derives from numb + skull. It is probably best use in self-deprecation, perhaps with a slight knocking on one’s own head.
The word means a state of resentful anger over accusations made. It does not benefit from having the adjective “high” precede it. And, with bludgeon and curmudgeon, no one really has any idea of its etymology.