I call it that because it’s even worse than a jerk . Like say the kanji is used in 20 compound words, and all 20 have the same general meaning (call it meaning A). Or it’s used in 20 words and in all of them it’s pronounced the same way (call it pronunciation A). The only exception is this ONE word that is pronounced in a fucked-up way and means something else (call it meaning B) totally unrelated. And guess what? In normal conversation, the ONLY TIME YOU ’LL EVER USE THE KANJI IS WITH MEANING ‘B’!
昨 is a typical COCK: It’s used in words like 昨月（さくげつ）、昨朝 （さくちょう）、昨日 （さくじつ）、 昨年（さくねん）、昨晩（さくばん）、昨報（さくほう）、and 昨夕（さくゆう）. But no one ever says those words. Instead, 昨 is only used in the context of 昨日（きのう） which is pronounced totally differently (ki instead of saku) and means ‘yesterday.’
Now, that’s not only hard to learn, but it raises quantum-physics-level epistemological questions regarding "what does this kanji mean?" If you go by "how many words use that meaning of the kanji", then Meaning A is the winner by 90%. But if you go by "How frequently is the meaning used in everyday life?" then Meaning B is the winner by 90%. How crazy is that??