Colloquial expression meaning ill-natured, nasty, or malicious; also used for situations or ailments that are persistent or particularly bad.
あの人は口では優しいけど、たちが悪いところがある。
That person seems kind in words, but has a nasty side.
最近の風邪はたちが悪くて、長引く人が多い。
The recent colds are nasty and tend to linger for many people.
タチが悪い冗談を言わないでくれ。
Don't make nasty jokes.
この汚れはたちが悪いから、専用の洗剤が必要だ。
This stain is tough to remove, so you need a special detergent.