Source : Webster's Revised Unabridged Dictionary (1913)

Cardboard \Card"board`\ (k[aum]rd"b[=o]rd`), n.
   A stiff compact pasteboard of various qualities, for making
   cards, etc., often having a polished surface.

Source : WordNet®

cardboard
     adj 1: resembling cardboard especially in flimsiness; "apartments
            with cardboard walls" [syn: {flimsy}]
     2: without substance; "cardboard caricatures of historical
        figures" [syn: {unlifelike}]
     n : a stiff moderately thick paper [syn: {composition board}]