If a Swede starts a sentence with fy, brace yourselves, because it's a word that shows strong emotions.
You might hear it uttered alone – Fy! – but it usually comes before another word, often a swear word or another noun understood as a profanity.
Some common examples include fy bubblan! ('oh, bubbles', roughly meaning 'my gosh!' – this one's quite safe to use), fy skam/skäms! ('Shame!' or 'Shame on you/him/them' etc), fy sjutton! (literally 'oh, seventeen!' –the number is often used as a tamer stand-in for more vulgar curses, but the origin is uncertain) or the most common, fy fan! (roughly 'damnit!' or 'for fuck's sake').
Beware: you might hear the last example a lot, but only use it when you're absolutely sure it's an appropriate situation, because this is quite strong language. A milder option often used is fy fabian, which replaces fan in the same way English speakers might exclaim 'Oh, sugar!' or 'fudge!' instead of swearing.
In Swedish you can say fy för snön. via GIPHY
But it's not always negative, and always depends on the context.
In colloquial Swedish, fy fan can be used to show surprise or excitement, in a similar way to 'oh my God!' in English, so for example fy fan, vad kul! just means 'Oh my God, how great!' and fy fan, vilken dag! means 'Oh God, what a day!' but isn't necessarily negative – it may have just been a surprisingly busy or eventful day.
On its own, fy serves to strengthen the words or phrases that come after it, so you might hear exclamations such as fy vad fint! (wow, how great!) or fy vad glad jag är! (wow, how happy I am).
You'll also hear it used a lot by pet-owners, in which case fy, Fluffy! means something like 'bad dog, Fluffy!'.
Fy has been around for centuries and existed in Old Norse too. And similar sounds exist, usually as a sound of disgust or anger, in many languages, including 'fie' in English, which has fallen out of use but was a popular interjection from the 13th century.
Å fy, vad äckligt!
Oh God, how disgusting!
Fy fan, vad vacker
Oh my God, how beautiful