Khus ka lainda is a Urdu swear word invented by Mehraz Ali Khan and popularized by Nawazish Ali Nusrat. The meaning of this word is unclear and vague but it is still a very cool thing to say.
ABC: Yaar, mera aaj phadda hai
XYZ: Kon khus ka lainda hai woh, main jaa ker peet-ta hoon usse
ABC: I have a fight today
XYZ: Who's that khus ka lainda? I'll beat him up.