The word is sinkadus, and it is used to refer to a specific roll of the dice, and more.
En sinkadus means a roll of the dice resulting in a five and a two. It comes from the French cinque et deux (literally 'five and two'), which entered Swedish through the Low German phrase sinke dûs, and was used to refer to this result in board games using two dice, such as old varieties of backgammon.
Today, it most often means 'chance' or 'stroke of luck', because of the random nature of any dice roll.
Two common ways to use it in a sentence are det var en sinkadus att… (it was a stroke of luck that) or … av en sinkadus (by chance). An alternative phrase would be av en slump, or using the adjective slumpmässig (randomly/by chance).
It doesn't always have to be positive; you could also use it to talk about something bad or neutral that happened due to sheer random chance, but sinkadus has a stronger positive connotation than slump.
And in the past, the word was also used to mean a slap or a punch, probably because of the link between the number five and five fingers on a hand. Poet Carl Michael Bellman used it in this way, writing about en väldig cinkadus på truten (that's 'a big slap around the mouth') in an 18th-century poem, but today this meaning is considered archaic.
If you find yourself wanting a colloquial word for 'slap', try örfil instead, which comes from the words öra (ear) and fila (to file). But be aware that in Swedish Finnish dialects, örfil means 'cinnamon bun', altogether a much nicer thing to give or receive.
Sinkadus has been recorded in Swedish since the mid-18th century, and in the meaning of 'chance' since at least the mid-19th. Today, variants also exist in Danish (sinkadus) and German (Zinkedus). It's not a word you're going to hear a huge amount in spoken Swedish, but sometimes it's the perfect word for the occasion.
Det var en ren sinkadus att jag fick jobbet!
It was a pure stroke of luck that I got the job!
Vi träffades av en sinkadus för två år sedan
We met by chance two years ago
Thanks to The Local reader Gillian Sjoedahl, who suggested sinkadus as our word of the day.