means Lacking taste or style in a way that's cheap, gaudy, or showily out of place — or, in the older literal sense, slightly sticky to the touch.

from Two senses braided together. The 'sticky' meaning is the simple one: it comes from 'tack,' the adhesive quality of not-quite-dry paint or glue. The 'in poor taste' meaning has a separate, earthier root — in 19th-century American Southern slang, a 'tacky' was a small, scruffy, worthless horse, and the word soon got flung at shabby, low-class people and things. The two streams have since merged in feel, so 'tacky' now suggests both the cheap and the unpleasantly clingy.