"Luca" is the common term in Chile for the 1000 Chilean Peso note, which is the lowest denomination of paper money in Chile, and which is worth about as much (usually) as a United States one dollar bill. As such, the term is very parallel to the word "buck" in the United States. And much like the word "buck", the term "luca" has an uncertain etymology dating back to the 19th century. The best explanation I have heard is that "luca" is short for "peluca", the Spanish word for "wig", because the famous historical figures on the notes had wigs. However, I can't find a reference for that story, and am passing it on as an urban legend.
Also, for me, the term shows just how diverse slang is in the Spanish speaking world: someone could spend a decade in Central America and never have an idea what the word "luca", used in Chile, means.