Bitterness is the sharp unpleasant taste of strong coffee or citrus peel. One of the five basic tastes.
Biologically it is a warning system, since many poisons are bitter, which is why children reject bitter food and adults slowly learn to enjoy it. The figurative sense is much commoner and it means lasting resentment, so bitterness after a divorce is anger that has settled and stayed. That is the sense in a bitter dispute. The adjective bitter covers both, and bitter is also a British beer named for its taste.
1nounbitternessA sharp, unpleasant taste.
“There was a real bitterness in the last mouthful.”
2nounangerLasting anger and resentment.
“Years of bitterness followed the split.”
Old English
biternes — bitterness; grief
Middle English
bitternesse
English, today
bitterness
From Middle English bitternesse, biternesse, from Old English biternes (“bitterness; grief”), equivalent to bitter + -ness.
Etymology from Wiktionary, CC BY-SA