Longman Dictionary of Contemporary English
Douglas Harper's Etymology Dictionary
n. 1 An experience or event that reveals, enlightens, or informs; something that causes learning, revelation, realization, or increased awareness. 2 An alcoholic beverage consumed first thing in the morning. A euphemism used by those offering or consuming alcohol in the morning.
Usage examples of "eye-opener".
On course we will: bootlegs and eye-openers by ther dozen we will dispense in honor of meetin' ye, at yer expense, an' don't ye fergit it, nuther.
He would also have poured down an eye-opener consisting of half a water glass of fine Champagne brandy.