1. First recorded use in medieval theological debates to mean someone who believed in human free will.
2. First political usage in 19th century france as a euphemism for anarchist when anarchist publications were banned.
3. Used since the mid 19th century onwards to diferentiate their proposed economic systems from those of classic socialists (ie libertarian communism, libertarian socialism etc). Worldwide this remains the most common use today.
4. Used since the mid-late 20th century to describe an extreme proponent of the free market. This use is almost unheard of outside the united states and canada.
It's real strange how these american radical capitalists use the word libertarian, seeing as how in the rest of the world it is usually followed by the word communist