A word used to describe a person who is bland, boring and/or stiff. Someone who shows no facial expressions and often times has no reaction to anything. Usually, Cardboards don Canada Goose jackets, wear their jeans around their ankles and peruse of the word "SWAGGA" excessively.
Troll 1: That guy is such a cardboard.
Troll 2: I know, I've never seen anyone as bland and expressionless as him.