skita i det blåa skåpet
Literally, "to shit in the blue cupboard". The original meaning behind this expression appears to be lost. Used as an expression in the blockbusting 80's comedy Göta kanal.
One theory is that the blue cupboard was the cupboard where the "not so rich" kept their finest belongings like the best porcelain, the finest table cloths, maybe the silver cutlery. Blue had been an expensive pigment/paint so when mass production of Prussian Blue started in the early 19th century this "luxury" colour became available to a wider group of the population. The blue paint, still being considered as a luxury item, was used for the finest furniture. The cheaper paints like red/red-brown/brown was used for the simpler things, like the potty cupboard. So, putting the non-empty potty in the blue cupboard was a major no-no. Answering a 'call of nature' directly in the blue cupboard would be an even greater social disaster, aside from being hugely impractical and uncomfortable.