Pronunciation of 上手 vs. 下手
For some one who does not know kanji pronunciation, I can guarantee that he/she will assume that the kanji character 手 in 上手 and 下手 are the same.
However, 上手 is pronounced じょうず and 下手 is pronounced へた
Both words share the same (hand character), one meaning (expert at/ good at) and the other meaning (bad at/unskilled at).
Why are they so different in pronunciation?
I know a Kanji can have multiple readings, but in the example above, the kanjis are used in a similar fashion, compound, so I assume we are using the ON-Reading for both of them, yet, there is no shared sound for the 手 character.
The 「じょうず」 reading seems to be originally derived from Chinese. The 「ず」 specifically appears to be a later change in the Japanese pronunciation. Another out-dated reading you'll see in dictionaries is 「じょうしゅ」 which is more reflective of the original Chinese. This is a fairly straight-up on-reading.
The 「へた」 reading of 下手 on the other hand is likely a case of "jukujikun" (熟字訓) which is a special type of reading of kanji based on the meaning rather than sound. So, the pronounced word 「へた」 has a different origin and kanji was chosen to represent it based on the meaning of the kanji. It seems fitting to choose 下手 to mean the opposite of 上手 in writing, even though the pronounced words have entirely different origins.
Welcome to the wonderful world of kanji. Try to relax and enjoy the ride. If you try to fight them with logic, the kanji will win.
The kanji always win.
The bigger picture: kanji are Chinese characters used to express Japanese. To accomplish this, Japanese sounds were assigned kanji based on corresponding meaning （訓読み）, (less often corresponding sound （当て字）. Chinese words were also imported and modified to be pronounceable in Japanese（音読み）.
Chinese characters were sometimes assigned to more than one Japanese word; 生 is a good example having multiple readings. Also, Chinese pronunciation is not the same across regions and eras. Some Japanese kanji thus have multiple Chinese readings (音読み) based on these pronunciations. The correct reading is based on where and when the word was learned. Essentially, it became a habit to say some words one way, and some words another.
Ultimately, this resulted in a writing system in which the correct reading depends on context, that is, the reader must decide what a combination of characters was intended to express. It's complicated enough that multiple interpretations are sometimes allowed.
Often, understanding the logic behind readings is an aid to understanding.
In this case, I think the best explanation is that 上手 and 下手 are logically constructed opposites in Kanji. The words are pronounced じょうず (音読み） and へた (訓読み[actually a 熟字訓]) in spoken Japanese.
The "下" included in "下手" is as a phonetic equivalent. The meaning is not opposite "上" . but "上手"="かみて" opposite meaning is "下手"="しもて".
e.g. "かみて" meaning is ”Stage Left” ”しもて” meaning is "Stage Right"
I don't construct the sentence in English well. Sorry. Could you understand above?
Thank you for the effort. I guess I can understand what you are trying to tell me.
How boring it would be if all the languages were regular, with no exceptions and their intricacies.
Consider yourself a lucky one, for Japanese has many mysteries for you to discover!