the.com/bad blood

The grudge that outlives the reason you started it.

means A lasting feeling of hostility or resentment between people, usually stemming from a past conflict that was never properly resolved.

from A straightforward metaphor dating back centuries: blood was long believed to be the seat of temperament and relationships, so 'bad blood' meant a corrupted bond between people. The same logic gave us 'blood feud' and the idea of blood as kinship — when blood between two parties turns 'bad,' it marks a rift that's personal and old, not fresh and fixable with an apology.

origin
Old term for feuds, literally believed to poison the blood
taylor swift
2014 song inspired by a real celebrity feud
psychology
resentment often outlasts memory of the actual insult
fun fact
Shakespeare used the concept before the phrase existed
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