Exactly!

> why do we need a word for throwing someone out a window?

Well, "defenestration" is a sort of inflexion of a common word (in Latin or German, which was a common language in Prague in 1618).

Wikipedia has quite a few examples, so having a specific word might be useful in the end: https://en.wikipedia.org/wiki/Defenestration

I think the answer about why it’s so specific is if you just say you threw someone out of a window people might assume it was only one story high and not lethal. Defenestration tells you that it was a high window, and often implies the ejection of the owner or a legitimate occupant. Similar to shock vs electrocution.

Even if (this is a challenge for the average English speaker like myself) you maintain in your conscious mind "fenestra = window", I think it's still so funny, because it sounds like you're removing the window from the person (a la decapitation).