Describes a person, behavior, or situation that is fundamentally unpleasant, mean-spirited, or troublesome. Often implies a deep-seated bad quality rather than a temporary mood.
Means 'mean-spirited' or 'malicious', often for deliberate unkindness. たちが悪い is broader, covering inherent bad nature or troublesome quality.
Etymology
The phrase combines たち (nature, disposition) with 悪い (bad). The exact origin of たち is uncertain, but it is used in expressions like たちがいい and たちが悪い to describe inherent character.
