Picture someone pointing back at a thing already named and saying on that. The word packs the pointing finger (there) and the word on into one piece, so the noun never has to be repeated. From that one move you can go to a real surface, to a topic treated as a surface, and to time treated as a line where one event lands on the next.
可以想成有人回頭指著剛剛講過的那個東西，說「在那上面」。這個字把那根手指（there）和 on 壓成一塊，於是那個名詞不必再說一次。從這個動作可以往三個方向走：真的表面、被當成表面的話題，還有被當成一條線的時間，一件事就落在另一件事上面。
From the word’s history
Recorded as Old English þǣron, the same parts as there + on.
來源記為古英語 þǣron，拆開就是 there 加上 on，跟 therein、thereof、thereto 是同一批字。
Wiktionary, "thereon"
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