Source : Webster's Revised Unabridged Dictionary (1913)

Eye opener \Eye opener\
   That which makes the eyes open, as startling news or
   occurrence, or (U. S. Slang), a drink of liquor, esp. the
   first one in the morning.

Source : WordNet®

eye opener
     n 1: an alcoholic drink intended to wake one up early in the
          morning
     2: something surprising and revealing