Think of a price tag with a number on it, and the number sliding down. That one movement — worth going down — is the whole word. Sometimes the number slides down by itself, the way a new car is worth less every month you own it. Sometimes a person slides it down on purpose: an accountant writing a smaller figure each year, or a writer talking about someone's work as if it were worth very little. What moves is always worth, and it always moves the same way: down.
Depreciate comes through Late Latin depretiare, built from de- (down) plus pretium (price) — the same pretium that gives English price. So the literal sense is to bring the price down, which is exactly the sliding number in the core image, and it explains why one verb covers both a car losing value and a speaker making something sound cheap.
depreciate 經中古英語 depreciaten，來自晚期拉丁文 dēpretiāre，由 de-（往下）加 pretium（價格）組成；英文的 price 也是從 pretium 來的。字面就是「把價格往下帶」，正好就是核心畫面裡那個往下滑的數字，也說明了為什麼同一個動詞可以同時管「車子掉價」和「把別人的付出講得很廉價」。
Wiktionary etymology supplied in this evidence packet (kaikki:3f6f69a190036ae4:315354627:20938): Middle English depreciaten < Late Latin dēpretiātus/dēpreciātus < dē- + pretium (“price”) + -ō.
Two branches
價值往下掉（錢）
這一家就是核心畫面最直接的樣子：價格上的數字往下。差別只在數字是自己滑下去的，還是有人一年一年把它往下寫。
verb · C1
If something such as a car, a machine or money depreciates, it becomes worth less than it was before, usually slowly and over a period of time. This use has no object after the verb.
貶值／跌價
把價值講低（話語）
同樣是往下，但掉的不是錢，而是「別人心裡覺得它值多少」。從價格搬到評價，是這個字唯一離開錢的一步。
verb · C1
To talk or write about a person, a piece of work or an idea in a way that makes it seem less important or less good than it really is; this use takes an object and is formal.
貶低／看輕