1. Having or being a taste that is sharp, acrid, and unpleasant.
2. Causing a sharply unpleasant, painful, or stinging sensation; harsh: enveloped in bitter cold; a bitter wind.
3. Difficult or distasteful to accept, admit, or bear: the bitter truth; bitter sorrow.
4. Proceeding from or exhibiting strong animosity: a bitter struggle; bitter foes.
5. Resulting from or expressive of severe grief, anguish, or disappointment: cried bitter tears.
6. Marked by resentment or cynicism: "He was already a bitter elderly man with a gray face" (John Dos Passos).
In an intense or harsh way; bitterly: a bitter cold night.
tr.v. bit·tered, bit·ter·ing, bit·ters
To make bitter.
1. That which is bitter: "all words . . . /Failing to give the bitter of the sweet" (Tennyson).
2. bitters A bitter, usually alcoholic liquid made with herbs or roots and used in cocktails or as a tonic.
3. Chiefly British A sharp-tasting beer made with hops.
1. having or denoting an unpalatable harsh taste, as the peel of an orange or coffee dregs Compare sour 
2. showing or caused by strong unrelenting hostility or resentment he was still bitter about the divorce
3. difficult or unpleasant to accept or admit a bitter blow
4. cutting; sarcastic bitter words
5. bitingly cold a bitter night
very; extremely (esp in the phrase bitter cold)
1. a thing that is bitter
2. (Miscellaneous Technologies / Brewing) Brit beer with a high hop content, with a slightly bitter taste
to make or become bitter See also bitters
[Old English biter; related to bītan to bite]