apron

Apron comes from Middle English napron, borrowed from Old French napperon, a diminutive of nappe meaning 'tablecloth,' which traces back to Latin mappa, 'napkin.'

The shift happened through rebracketing: 'a napron' was misheard and reanalyzed as 'an apron,' so the n slid over and stuck to the article instead of the noun.

It shares this exact kind of slip-of-the-ear reshaping with words like adder, newt, and umpire, which also lost or gained a letter by trading it with 'a' or 'an.'