noun
nature; disposition; temperament
Refers to a person's inherent character or temperament. Often used in phrases like たちが悪い (bad-natured) or たちのいい (good-natured).
彼はたちが悪いから気をつけて。
He has a bad nature, so be careful.
あの人はたちがいいね。
That person has a good disposition, doesn't he?
noun
Describes the inherent quality or type of something, not just people. Often used in set phrases like たちの悪い病気 (a nasty illness).
この風邪はたちが悪くて、なかなか治らない。
This cold is a nasty one and just won't go away.
The origin is uncertain. It may be related to the noun 質 (たち) meaning 'quality' or 'nature', but the exact derivation is unclear.