From English 'dirty'. Used colloquially to mean dirty, underhanded, immoral, or illegal (often functions like a na-adjective).
彼は試合でダーティーな手を使ったと非難された。
He was accused of using dirty tactics in the match.
ダーティーなビジネスには関わりたくない。
I don't want to be involved in dirty/underhand business.
あの映画はダーティーな展開が多くて、大人向けだ。
That movie has a lot of dirty/unsavory developments and is meant for adults.