This is my favorite WTF image in a long time. Dating from 1555, it’s a Renaissance image. I don’t officially know the story behind it, but there’s a good discussion of it on Reddit.
‘Flaisch macht Flaisch’ is from the old German proverb ‘Fleisch macht Fleisch, fisch macht nisch’. It would roughly translate to ‘meat gives meat, fish gives nothing’, meaning that meat is more nourishing than fish.
Fleisch is in German also connected to ‘Fleischeslust’, which means carnal desire. The fish can be connected to the catholic tradition of eating fish on friday, or abstinence from eating meat.
So in this picture, the proverb is interpretated (sic) in a raunchy way. The nun wants to trade the fish that implicates abstinence from meat/lust for the meat that obviously represents carnal desire.
Considering the subject, time period (1555) and origin (Germany, as indicated by language), it is presumably Protestant propaganda. Nuns and monks were widely depicted as subsumed by material desires in contrast to their vows, and Protestants had rejected celibacy as a requirement for priests. Quite the opposite, they commonly emphasized the importance of marriage, children and family bonds.
Thus, the nun is desperate to “get some” as a woman would naturally want, presumably from the leering man in the jester’s cap hanging out the window, who has just been emasculated. In a further bit of satire, a phallus also dangles from her rosary..