I've also heard it as "knock me for a loop", and I think it calls back to "feeling loopy" as a sort of "dizzy", so to throw/knock someone for a loop is to metap

I've also heard it as "knock me for a loop", and I think it calls back to "feeling loopy" as a sort of "dizzy", so to throw/knock someone for a loop is to metaphorically hit them so hard as to make them dizzy or disoriented.