Q:

Why does the prefix “auf-” change the word “hören” (hearing) to mean stop?

A friend of mine who’s learning German asked me this and I couldn’t explain it to her. Is it just an example of how a word changes over time?
Perhaps it used to me “hear me tell you to stop”, “hör auf [my command]”?

A:

Duden says mittelhochdeutsch ūfhœren, eigentlich wohl = aufhorchend von etwas ablassen

to stop (making noise) and prick (up) one's ears

but that etymology has no safe origin. So, we don't know. 

