bad blood (uncountable)

1. (idiomatic) Feelings of hostility or ill will.
   • 1922 February, James Joyce, “[[Episode 16]]”, in Ulysses, Paris: Shakespeare & Co.; Sylvia Beach, OCLC 560090630 ↗; republished London: Published for the Egoist Press, London by John Rodker, Paris, October 1922, OCLC 2297483 ↗:

     All those wretched quarrels, in his humble opinion, stirring up bad blood, from some bump of combativeness or gland of some kind, erroneously supposed to be about a punctilio of honour and a flag, {{...}

2. (idiomatic) A serious feud or long-standing grudge.
3. (idiomatic, dated) An inherited immoral or disturbed nature.

   • "No!" was the determined answer. . . . Because his father was dishonest is no proof that he is a thief."

• (hostility, ill will) animosity, animus
• (feud) blood feud, vendetta

• Spanish: mala sangre, mala leche, mala uva