Futsukushii is believed to be formed by mingling the "u" in "utsukushii" with a breathe of sigh, hence "futsukushii".
Though it is thought to be a common case of typo or a net slang, the origin seems to actually originated from the anime series 「Yu-Gi-Oh! Duel Monsters」, where Seto Kaiba used that word in one of the episode (thought to be a misheard).
In pixiv it is commonly used as a tag praising artworks that are thought to be beautifully drawn. Though it's also common to use this tag for Yu-Gi-Oh! related illustrations.
While it is usually used as a template tag for praising artworks, in the end it's tagging criteria are leaning more towards an artwork that are deemed worthy of this tag by the tag editors subjetively; hence there are a lot of bias cases from the editors towards people who submitted particular illustration that they think it deserved.
Though it is also possible to tag your own illustrations with this tag, it might cause a lot of displeasing comments from the viewers to the point of flaming. So you are advised NOT to do so.
The pros. and cons.
It is originally an evaluation based tag where it is meant to spark up a battle of "attach or detach" game. Interestingly, there are also cases of outbursts where people replace the tag with "Utsukushii 美しい" instead of the former.
There are also various reasons like "Since it's just disgusting might as well just use "美しい" instead" or "It might cause people to think it as "腐つくしい (the kanji 腐 means rotten)". But in return, there are also opinions like how using "美しい" might be so straight foward to the point it's embarrassing.
Regardless of its cons., pretty illustrations that makes you sigh a breathe of relief does exist.
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