D defs.my
Entry 2 senses Webster, 1913

eye opener

· IPA
01 n. That which makes the eyes open, as startling news or occurrence; something which causes one to suddenly understand what was not previously known.
  1. 1.
    That which makes the eyes open, as startling news or occurrence; something which causes one to suddenly understand what was not previously known.
  2. 2.
    A drink of liquor, especially the first one in the morning.[U. S. Slang]