Lake, meer means lake. Achtermeer is best translated as back lake, or behind lake. Assuming achter in this case is used as this. It could also mean the lake of Acht. Since Acht could also be the name of a location. See Markermeer.
I think what @i_love_FFT@lemmy.ml means is that on the Ottoman map you kinda get France, and then directly on the coastline right north of France you get Jutland. It's sorta like if you took Europe and did a ripple cut to remove the Netherlands out of it.