The Collaborative International Dictionary of English v.0.48:

Dingy \Din"gy\ (d[i^]n"j[y^]), a. [Compar. Dingier; superl.
   Dingiest.] [Prob. fr. dung. Cf. Dungy.]
   Soiled; sullied; of a dark or dusky color; dark brown; dirty.
   "Scraps of dingy paper." --Macaulay.
   [1913 Webster]