wormwood

noun2

1. •
   An intensely bitter herb (Artemisia absinthium and similar plants in genus Artemisia) used in medicine, in the production of absinthe and vermouth, and as a tonic.

   • But as I said, / When it did taste the wormwood on the nipple / Of my dug and felt it bitter, pretty fool, / To see it tetchy and fall out with the dug! — 1591
2. •
   (figurative) Something that causes bitterness or affliction; a cause of mortification or vexation.

   • The irony of this reply was wormwood to Zeluco; he fell into a gloomy fit of musing, and made no farther inquiry […]. — 1789