The im and the tu are not blameless: a bit more assertiveness from them and the loathsome promp might explode less hideously into our sentences, but it's the promp that's the main problem.
If the word meant person who barges to the front of the crowd and demands to be served first then, fair enough, the word might be appropriate.
But an impromptu (ouch, isn't it horrible?) is a lovely thing, a first fine careless rapture. Something created without forethought, begun without conscious effort, a gift from...well, whether Nature, God, Experience, Madness or simply Joy, who can say.
Sometimes they're miraculous.
But I do wish they weren't called...
...well, you know what.
Word Not To Use Today: impromptu. This word arrived in the 1600s from France, and before that it comes from the Latin in promptū, which means in readiness.
Which is, I suppose, in some ways the exact opposite of what it means today.