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1121, from up + on, probably influenced by O.N. upp a. Distinct from O.E. uppan which merely meant "up."
In the mod. Scand. tongues, except Icelandic and Færöese, the reduced form pa, paa, corresponding to Eng. (colloq. or dial.) 'pon, 'po', has displaced the simple prep. å, aa = on. [OED]