Bitters

a liquid, often an alcoholic liquor, in which bitter herbs or roots have steeped, used as a flavoring, especially in mixed drinks, or as a tonic.
Pharmacology.

a liquid, usually alcoholic, impregnated with a bitter medicine, as gentian or quassia, used to increase the appetite or as a tonic.
bitter medicinal substances in general, as quinine.

having a harsh, disagreeably acrid taste, like that of aspirin, quinine, wormwood, or aloes.
producing one of the four basic taste sensations; not sour, sweet, or salt.
hard to bear; grievous; distressful:
a bitter sorrow.
causing pain; piercing; stinging:
a bitter chill.
characterized by intense antagonism or hostility:
bitter hatred.
hard to admit or accept:
a bitter lesson.
resentful or cynical:
bitter words.
that which is bitter; bitterness:
Learn to take the bitter with the sweet.
British. a very dry ale having a strong taste of hops.
to make bitter:
herbs employed to bitter vermouth.
extremely; very; exceedingly:
a bitter cold night.
Contemporary Examples

The Absinthe-Minded Porteños of Buenos Aires Jeff Campagna March 9, 2014
The Rise and Fall…and Rise Again of the Old-Fashioned Allison McNearney June 13, 2014
The Absinthe-Minded Porteños of Buenos Aires Jeff Campagna March 9, 2014
Lights, Camera, Cocktails Brody Brown September 1, 2011
The Secret Speakeasies of Buenos Aires Jeff Campagna February 24, 2014

Historical Examples

Out of the Hurly-Burly Charles Heber Clark
Tonio, Son of the Sierras Charles King
How Private George W. Peck Put Down The Rebellion George W. Peck
Cruel As The Grave Mrs. Emma D. E. N. Southworth
Dangers of the Trail in 1865 Charles E Young

plural noun
bitter-tasting spirits of varying alcoholic content flavoured with plant extracts
a similar liquid containing a bitter-tasting substance, used as a tonic to stimulate the appetite or improve digestion
adjective
having or denoting an unpalatable harsh taste, as the peel of an orange or coffee dregs Compare sour (sense 1)
showing or caused by strong unrelenting hostility or resentment: he was still bitter about the divorce
difficult or unpleasant to accept or admit: a bitter blow
cutting; sarcastic: bitter words
bitingly cold: a bitter night
adverb
very; extremely (esp in the phrase bitter cold)
noun
a thing that is bitter
(Brit) beer with a high hop content, with a slightly bitter taste
verb
to make or become bitter
n.
adj.

bitter end
bitter pill to swallow