Source : Webster's Revised Unabridged Dictionary (1913)

Dandruff \Dandruff\, n. [Prob. from W. toncrust, peel, skin +
   AS. dr?f dirty, draffy, or W. drwg bad: cf. AS. tan a letter,
   an eruption. [root]240.]
   A scurf which forms on the head, and comes off in small or
   particles. [Written also {dandriff}.]

Source : WordNet®

dandruff
     n 1: a condition in which white scales of dead skin are shed by
          the scalp
     2: loose scales shed from the scalp; "I could see the dandruff
        on her shoulders"