Meaning

adverb with bitterness, in a resentful manner

adverb indicating something hard to accept

adverb extremely and sharply

originFrom Middle English bitterli, biterli, biterliche, from Old English biterlīċe, bitterlīċe (“bitterly”), equivalent to bitter + -ly. Cognate with German Low German bitterlik (“bitterly”), German bitterlich (“bitterly”), Swedish bitterligen (“bitterly”).

Etymology from Wiktionary, CC BY-SA 3.0