According to seatlepi.com:

There’s a story going around South Lake Union, but a spokeswoman for Vulcan, Paul Allen’s development company, says it’s just an urban legend.

That aside, the story that the neighborhood’s streetcar line now under construction was called the South Lake Union Trolley until the powers that be realized the unfortunate acronym — SLUT — seems here to stay.

Officially, it’s now the South Lake Union Streetcar. But the trolley name already has caught on, and in the old Cascade neighborhood in South Lake Union, they’re waiting for the SLUT.

At the Kapow! Coffee house on Harrison Street, they’re selling T-shirts that read “Ride the SLUT.”