The word 'upon' is a classic example of grammaticalization and fusion in the history of English. It originated in Middle English as a simple combination of two common, closely related prepositions/adverbs: 'up' and 'on'. This combination served to add emphasis or a specific directional nuance, meaning essentially "up and on" or "on top of." Over centuries of frequent use, the two words fused into a single, univerbated form, 'upon'. While its core meaning remains closely tied to its constituent parts, its usage has expanded into more abstract and temporal contexts, such as "upon arrival," reflecting the natural evolution of prepositions in the language.

This is a fused compound word from Middle English, formed from the adverb 'up' and the preposition 'on'. Both elements are of Old English origin: 'up' (upp) and 'on' (on).