From Old French 'nonper' (not equal, neutral party), misdivided from 'a nompere' to 'an ompere' — losing its initial 'n' to the article.
An official who watches a game or match closely to enforce the rules and settle disputes.
From Old French 'nonper' (not equal, peerless — i.e., a neutral third party), from 'non' (not) + 'per' (equal, peer). 'A nompere' was misdivided into 'an ompere,' then became 'umpire.' The initial 'n' was stolen by the article, just like 'an apron' from 'a napron.' Key roots: non (Latin: "not"), par (Latin: "equal").
An 'umpire' is a stolen letter away from 'nonpareil.' The original word was 'nompere' (not-equal, a neutral third party). But English speakers misheard 'a nompere' as 'an ompere' — the 'n' jumped from the word to the article. This same n-migration gave us 'apron' (was 'napron'), 'adder' (was 'nadder'), and 'orange' (was 'naranj'). English has been mishearing word boundaries for a thousand years.