The word 'thereupon' is a compound formed in Middle English by directly combining two existing adverbs, 'there' and 'upon'. 'There' originates from the Proto-Germanic þa, a demonstrative pronoun root meaning "that", which also gives rise to related words like 'that' and 'the'. 'Upon' derives from the Old English uppan, a fusion of up (up) and on (on). The compound's literal sense, "on that (thing or event)", evolved through logical extension. It first described a physical position ("on top of that"), then shifted to a temporal and consequential meaning, indicating something happening immediately after or because of a preceding event, capturing the sequence where one thing rests upon another.

Structure: there + upon - there: From Old English þǣr, þār (in or at that place). An adverb indicating a specific location or point. - upon: From Old English uppan (on, on top of, up to). A preposition indicating position or relation in time or sequence.