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The Finnish word "Sopia" means "fit" , "kuin" is subordinating conjunction and means "like" , "nenä" is "nose" and "päähän" means "in head". Literally this sentence would be "Fit like a nose in head". It means that something looks very good with something, as good as nose in head. In real life you might ask your friend, does this dress look good on you, and your friend says, that it looks wonderful, fit like nose in head, so then your friend means, that its perfect. The English expression "fit like a glove" means precisely the same as our expression, but just with a one different word. I find this phrase is pretty logical. SOPIA KUIN NENÄ PÄÄHÄN
In Spanish there is a similar expresión but in this case it plays with the Word ANILLO (Ring) and DEDO (finger) so it is also logical as a ring has to fit in your finger or you can lose it. SENTAR COMO ANILLO AL DEDO