Source : Webster's Revised Unabridged Dictionary (1913)

Denim \Den"im\ (d[e^]n"[i^]m), n. [Of uncertain origin.]
   A coarse cotton drilling used for overalls, etc.

Source : WordNet®

denim
     n 1: (usually plural) close-fitting pants of heavy denim for
          casual wear [syn: {jean}, {blue jean}]
     2: a coarse durable twill-weave cotton fabric [syn: {dungaree},
         {jean}]