The Collaborative International Dictionary of English v.0.48:

Denim \Den"im\ (d[e^]n"[i^]m), n. [Of uncertain origin.]
   A coarse cotton drilling used for overalls, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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