The term "world-line" is a little odd in English. Google n-grams shows the English term going back to 1915 in the books google has scanned. Is its origin in Minkowski, Raum und Zeit (1909), Jahresberichte der Deutschen Mathematiker-Vereinigung, 75-88 ( http://de.wikisource.org/wiki/Raum_und_Zeit_%28Minkowski%29, English translation at http://en.wikisource.org/wiki/Space_and_Time_%28Prasad%29 )? The Minkowski paper seems to make it somewhat more understandable. Although the title of the paper translates as "space and time," in the text it seems that Minkowski uses "world" to mean "spacetime:"
Die Mannigfaltigkeit aller denkbaren Wertsysteme x, ,y, ,z, ,t soll die Welt heißen.
In Prasad's translation:
The totality of all conceivable systems x, y, z, t may be called the world.
So it seems like Minkowski didn't coin "Raumzeit," "spacetime," and instead used "world" to mean "spacetime," so that "world-line" means "spacetime-line." Have I got this right historically?