there-up-on / thâr-ə-pŏn, thâr-ə-pôn
1. concerning that; referring to that; upon that
Living at a river, one comes to know the nature of the fish therein; Dwelling by a mountain, one learns to recognize the language of the birds thereupon.
2. immediately following
Thereupon he began to pray to Hercules, without other exertion.
From “Hercules and the Carter” a Fable by Aesop, 620 BC – 560 BC
3. as a result
If we conceive that anyone loves, desires, or hates anything which we ourselves love, desire, or hate, we shall thereupon regard the thing in question with more steadfast love, etc.
Baruch Spinoza, 1632 – 1677
A combination of the English adverb there and the English adverb and preposition upon. The adverb there comes through Middle English from the Anglo-Saxon adverb thaer of the same meaning, which is akin to thar, ther, and dar in other Germanic languages. Upon is a combination of two English adverbs/prepositions, up from the Anglo-Saxon up/uppe and on from the Anglo-Saxon an/on, all of the same meanings. Up is related to other Germanic adverbs/prepositions like Old Norse upp, Dutch op, and German auf, and on is related to the German adverb/preposition an (on) and the Greek adverb/preposition ana (on, upon, up, up to).
Thank you to Allen Ward for providing this etymology of thereupon.