How to find earliest usage of a term?
This question could be split into two types of terms: new expressions &amp; old expressions.

I would rather split it into the following:

Textual usage in the classics &amp; history, and later
Pre-Qin paleographic occurrences

While resources for the former are abundant (resources like 漢語大詞典 and later will give a good survey, even if not the earliest usage), resources for the latter are specialised and scarce. This is because literature of the former category are basically canonicalised, while texts in the latter category are still a fresh, hot area of research.
For a searchable database of the latter category,

For words, see 先秦甲骨金文簡牘詞彙庫.
For single characters,

The average user should look at 漢語多功能字庫 - this gives detailed paleographic explanations for each entry

If you have a  working knowledge of Pre-Qin script shapes, you can use 小學堂. Note that each entry in 小學堂字形演變 also links to a 漢語多功能字庫 entry if it exists.

小學堂字形演變 does not aim to give accurate glyph origin explanations. The character explanations given directly underneath the table are mainly from well-known or canonicalised books for reference purposes, and may or may not be the modern professional consensus.
The aim of 小學堂字形演變 entries is to provide a professional consensus on the historical forms of a character and related characters, presented in a table. For the actual explanations behind a character, please see the 相關索引 section underneath each entry for the references.





Worked example for words on 先秦甲骨金文簡牘詞彙庫: Earliest occurrence of the term 中國

Go to 先秦甲骨金文簡牘詞彙庫

Click on 詞彙檢索資料庫


Type 中國 in the box labelled 詞彙. Optional filters include (Leave blank to not apply any filter restrictions):

Part of speech (詞類)
Resource type:

Oracle bone script (甲骨文) and associated literature
Bronze inscriptions (金文) and associated literature
Bamboo and wooden slips (簡牘)




Click 查詢

Only one result comes up:

The citation (書籍 &amp; 編號) is 殷周金文集成 6014.

嚴式 &amp; 寬式 refer to strict and loose spelling: Since 國 did not exist originally, the word representing 中國 was originally written as 中或. Thus, 中或 is the strict spelling of the word as it appears in the inscription.


殷周金文集成 is hosted on 國學大師, and you can search record #6014 there:


Have fun deciphering! E.g. analysis of a short fragment:


隹（唯）珷（武）王旣（克）大邑商（則）【⿺⿹人彡】（筳）（告）【⿰于弓】（于）天曰余（其）宅（兹）（中）或（國・域） 自（之）【[⿰]】（乂）【⿸目十】（民）
For (唯) King Wǔ (武王) had already (旣) defeated (克) the Great (大) State (邑) of Shāng (商). He thus (則) conducted ritual divination (筳 &gt; 筳篿) and prayed (告) to (于) Heaven (天), saying (曰): 

I (余) shall (其) settle (宅) in these (兹) central (中) regions (或 &gt; 國・域), and govern (乂) the people (民) from (自) here (之).