Here are some synonyms for unprepossessing:
* Plain
* Unremarkable
* Uninteresting
* Uninspiring
* Unattractive
* Unimpressive
* Drab
* Unassuming
Examples:
* "The hotel was unprepossessing, with faded paint and peeling wallpaper."
* "He was an unprepossessing man, with a receding hairline and a tired expression."
* "The movie's plot was unprepossessing, lacking any real originality or excitement."
The word often implies a lack of charm or sophistication, making it a slightly negative connotation.