from world english dictionary:

-adj
1. Compare sour having or denoting an unpalatable harsh taste, as the peel of an orange or coffee dregs
2. showing or caused by strong unrelenting hostility or resentment eg: he was still bitter about being a minimum wage grunt
3. difficult or unpleasant to accept or admit: a bitter blow
4. cutting; sarcastic: bitter words
5. bitingly cold: a bitter night