noun
Used in the pattern 〜も骨頂だ to mean 'the height of ...' or 'the ultimate in ...', often with a negative or critical nuance.
そんなことをするなんて、無礼も骨頂だ。
Doing such a thing is the height of rudeness.
彼の態度は怠慢も骨頂だ。
His attitude is the pinnacle of negligence.
極み also means 'height' or 'pinnacle', but is more formal and often used in positive contexts, while 骨頂 is commonly used in the pattern 〜も骨頂だ with a critical tone.
The kanji 骨 (bone) and 頂 (top) combine to suggest the 'topmost bone' or 'core', figuratively meaning the extreme or pinnacle of something. The reading こっちょう is a regular on'yomi compound.