This is one of my favorite Swedish-isms. It comes from the 1950s American TV series about a dog (Lassie). In the 90s, a Swedish band made a song called "Jag mötte Lassie" which alludes to encountering the dog from Lassie. This prompted an OP-ed in one of the two national tabloids where a journalist listed others' weird or funny encounters with celebrities. The one that stuck out was a woman whose family in the US had a dog that had been bred by Lassie (Lassie is a bitch in the series, but was frequently played by male dogs). People started sending in their own recounts, which prompted the tabloid to run a regular feature with the readers' wildest stories about their random encounters with a celebrity. That term lived on, and today you can hear Swedes sometimes telling their best "Lassies". The term is loosely related to urban legends, as a "true" Lassie should be a third person recount of someone meeting a celebrity in an unexpected way or setting.