E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
This is a strange blunder. A napperon, converted into An apperon. Napperon is French for a napkin, from nappe (cloth in general). Halliwell, in his Archaic Dictionary, p. 571, gives Nappern (an apron) North.
Other examples of n attached to the following noun, or detached from it, are an adder for a nadder (Old English, nddre); a newt for an ewt; a nag (Danish, ög); nuncle (Shakespeare), mine uncle; For the nonce (this once), where n is transferred from the preceding pronoun tha-n or the-n, i.e. this-n (accusative case after for).