Not fashionable, interesting, or socially acceptable; something that makes you seem unhip or out of touch with what's considered cool.
Prefix 'un-' (not) plus 'cool,' which evolved from slang meaning 'socially acceptable' or 'impressive,' possibly originating in African American vernacular in the mid-20th century.
What's 'uncool' changes constantly—Bell bottoms were cool, then uncool, and now they're cool again—this shows that coolness is entirely about what social groups agree on, not about any quality in the object itself!
Complete word intelligence in one call. Free tier — 50 lookups/day.