Modern German is a language that has its origin in a huge amount of different germanic dialects which again can be grouped together to clusters where linguists still discuss if they are distinct languages or dialects of a common language. The main German sub-languages are:
- Niederdeutsch (low German) spoken in the northern flatlands.
- Hochdeutsch (high German) spoken in the alpine south.
Hochdeutsch is subdivided in Mitteldeutsch (middle German) and Oberdeutsch (upper German). But be careful! The word Hochdeutsch has a second meaning: In other contexts it is a synonym for Standarddeutsch (standard German) which is not a group of dialects, but the name of the language you are learning now.
So, at the coasts of North Sea and Baltic Sea Low German was spoken, and there they caught this funny flat and round fish with both eyes on the same side of it's head (similar but not identical to flounders), and in the 16th century the Germans imported the name »Butt« from the Dutch language that is very close related to Low German. The ancestors of both languages, Middle Low German and Middle Dutch, share a rather big amount of their vocabulary, and in this era there also was this adjective in both languages:
- "butt", "bot" = blunt, dull, stub, stump, butt
As you can see here, in English the adjective butt survived until today, but also the English noun "butt" (buttocks, stump end, stub, ...). And there is also an English verb that derived from the same origin: to beat.
This ancestor is a Indo-European word that sounds like bhaud, bhaut, bhud, bhut and means to blunt, to bulge, to beat.
So, the word Butt was part of Low German, but not of High German. It only spread to the south when it became possible to transport frozen fish to southern regions, and then Butt became known as the name of an exotic flat fish, but in the south of Germany and in Austria and Switzerland this word never did mean blunt. It only was a strange name of a strange fish.
But in the south there was a wicker basket in use, that you could carry on your back, and this basket has the name »die Bütte« or »die Butte«. But it has a completely different etymology. (I live in Austria, and when I heard the name Butt for a fish when I was a young boy, I wondered how it was possible, that a fish was named after a wicker basket, but of coarse I was wrong.)
So, the word »Buttkopf«, »Butkopf« or »Butskopf« just means "blunt head, dull head".
Maybe you remember the cartoon series Beavis and Butt-Head from the 1990ies (1993-1997)? The English name "Butt-Head" has exactly the same etymology (but not the same meaning!) as German »Butskopf«.