In total around 8% of the spanish dictionary is derived from arabic.[1]
[0] http://en.wikipedia.org/wiki/Arabic_language_influence_on_th...
[1] http://cvc.cervantes.es/ensenanza/biblioteca_ele/asele/pdf/1...
Adder, apron and umpire all used to start with an "n". Constructions like "A nadder" or "Mine napron" were so common the first letter was assumed to be part of the preceding word. Linguists call this kind of thing reanalysis or rebracketing.
Apparent counter-evidence: the relevant Portuguese definite article is "a" rather than "la" as in Spanish.
Counter-counter-evidence: at least one source (W V O Quine's "Quiddities") tells me that once upon a time the Portuguese definite article was "la" rather than "a". However, Quine was a philosopher rather than a historian of language, this particular book is a fairly frivolous one, and I don't know how far I can trust him.
Counter-counter-counter-evidence: A few minutes of googling haven't found me any other evidence for Portuguese, or any other language spoken in Portugal, ever having had "la" as a definite article.
Make of all that what you will. I'd be very surprised if it weren't derived from a definite article prefix, but exactly how it happened I would rather not guess.