One equal temper of heroic hearts,
Made weak by time and fate, but strong in will
To strive, to seek, to find, and not to yield.
- Tennyson, Ulysses
Later it came to mean "the last or highest term of a rhetorical climax" and "the highest point of anything reached by gradual ascent; the culmination". The OED says that these latter two meanings "are due to popular ignorance and misuse of the learned word". Who says the OED is a descriptive dictionary?
Greek κλῖμαξ meant "ladder". It's from Proto-Indo-European *ḱlei- "to lean". The suffixed o-grade form *ḱloi-tr- became Old English hlǣdder, then English ladder. But what is that *-tr- supposed to be?