A bathrobe was a loose-fitting robe worn before and after bathing or using a steam room.
This type of clothing was mentioned in a detective novel Data was reading in early 2364 to become more prepared for a Dixon Hill adventure. (TNG: "The Big Goodbye")
While playing cards with his band one night, Vic Fontaine recalled a story about Dean Martin and Frank Sinatra, which Fontaine revealed mid-story "…but this time he's switched all the robes in the steam room. So when Dino goes to pick up his robe, he doesn't realize that across the back, Frank has printed in big bold letters…", but did not finish, as he was interrupted by Ezri Dax. (DS9: "It's Only a Paper Moon")