>From the Compact Oxford English Dictionary, 2d ed.
libertarian:
...2. One who approves of or advocates liberty.
1878 Seeley Stein III. [doesn't give anything else].
1901 F.W. Maitland in English History Review July 419. ...In such
matters Englishmen are individualists and libertarians. The picture of
an editor defending his proof sheets ... before an official board of
critics is not to our liking.
There's other cites from 1906, 1966, 1969, and 1972.
The archaism "libertine" may in certain contexts be synonomous with
"libertarian." I think this word is much, much older -- Roman Empire or
thereabouts.