Lacking the ability to impress, inability to produce an impression.(adjective)
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Use "unimpressive" in a sentence
"In between, there's a confused tale and a couple of sequences filmed in unimpressive 3D."
"At the time I thought it looked great, because it was our first glimpse, and I still think the film will be good (read my post above), but why I say it was unimpressive is because I've realized that the trailer wasn't really as amazing as I originally thought …"
"This is really worse than the dentist if you have thin unimpressive hair like mine."