Is there a reason why [中]{ちゅう}[古]{こ} means used/second-hand items ?

Question

I am learning Kanji using Kanji Damage deck on Anki and I came across this vocabulary for 古:

[中]{ちゅう}古{こ} の XXX : used XXX

If I try to literally translate 中古:

[中]{ちゅう}(middle/central) + 古{こ}(old) = middle old ?

After a quick search on wictionary, I found that one of the etymology for the word says it is Japanese invention (link).

Wasei kango (和製漢語), borrowing from Japanese [中]{ちゅう}古{こ}‎ (chūko, “secondhand”)

Is there a reason why 中 is used with 古 here while [古]{ふる}[着]{ぎ} means old/used clothes without any need of 中 ?

Answer 1

While it does mean middle ages in historical context when you pronounce it ちゅうこ{HHLL}, the "secondhand" definition (read ちゅうこ{LLHH}) is unrelated to that.

In this word 中【ちゅう】 stands for "not completely (being one side); so-so":

２ 程度・価値・等級・序列などがなかほどであること。良くも悪くもないこと。

So 中古 literally means "half-old", that is not worn down to a nub, still usable. Historically the reading ちゅうぶる is first attested, but it's almost dying out as far as I know.

Is there a reason why 中 is used with 古 here while 古着 means old/used clothes without any need of 中 ?

That's complicated, but mostly because it's an established word. Not so many words attached by 古- actually mean "secondhand". 古着, 古本 and 古道具 maybe? Otherwise they'll mean really old.

古本、中古コミック、中古漫画の買取や購入は日本最大級のコミック通販サイト ネットオフをご利用ください。 (Source)

FYI there is a derived word 新古 to mean un-used goods that aren't able to be sold as new stocks for some reason.

Answer 2

"中古" means just used but usable with or without renovation.

"中古" is most commonly used for the used but still usable things.

"中古住宅"、"中古車"、"中古マンション（We Japanese call just コンドミニアム as マンション）"、”中古パソコン”

"中" of "中古" perhaps indicates the condition of the thing that is once used but still usable.