PeriodicallyPedanticEnglish is such a peach.
Phil KIt’s because the im isn’t a prefix but part of the word. (It was originally spelt empeche)
Looking at the etymology, impeach is from old-french “empecher”.
Except that “em” was an alternative form of the old-french prefix “en”.
And that “empecher” is itself is derived from the late latin “impedicare”, which uses the Latim prefix “im” from which the French prefix comes. And is prefixed to “predica”.
Of course, the latin (and French) prefixed aren’t used to indicate opposite meaning, like “in” often is. But that’s just yet more bullshit.
It’s basically from back in the day when there were still dinosaur peaches large enough to contain a badly-behaving consul.