A term used as a comeback when you cant think of any comeback much like the term "f**k off". Commonly used by many New Zealander's.
Basically like the new "f*ck off"
Whatevs, comes from what ever and Lang Lang is just a random word used to make the whole statement seem more fuller and have more impact by repeating a word.
Adam: Your hair looks ranga (ginger)
Josh: Whatevs Lang-Lang