This is one of the most sneaky pairs of false friends and you can go years without ever noticing that you got it wrong. That’s what happened to me at least.
But while the English eventually means that something WILL happen, even if it takes a while, the German eventuell actually means…. maybe.
- Morgen habe ich eventuell Zeit.
- Tomorrow, I might have time.
The origin of the words (and other words like event itself) is the Latin evenire, which meant something like to come out, to happen.
So that’s in line with the English mean, but German, for some reason, shifted toward being doubtful.
By the way… the proper translation for eventually highly depends on context. Endlich is a good pick, but in some contexts, irgendwann is the more idiomatic choice.