A feeling of anger, hate, hostility between individuals or groups due to past events, leading to serious feuds

There's a lot of bad blood between their families, but it's a story going back a long time and nobody remembers the why and wherefore anymore

Non corre certamente buon sangue tra le loro famiglie, ma è una storia di vecchia data e nessuno ricorda più il perché e il percome

As children they were inseparable, but now there is bad blood between them to the point that they haven't spoken to each other in ten years

Da bambini erano inseparabili, ma oggi c'è della vecchia ruggine tra loro tanto che non si parlano da almeno dieci anni

Bad blood is based on the old idea correlating blood and emotion. The current expression replaced, in the early 1800s, the previous version Ill blood