Although, there is no obvious English cognate, amargo is the Spanish word for bitter. Bittersweet, for example, is amargodulce: literally, bitter-sweet.
Interestingly, though, the very common Spanish word for “yellow,” amarillo, comes from this same root for bitter. It literally means “a bit of bitterness,” from the Latin amarus for “bitter” with the –illo diminuitive ending.
Yellow — the color of melancholy, of puke, of snot — is really the color of just a hint of bitterness.