The phrase was originated on the forum, "The Pit", by (now) esteemed user, Stratoblaster73, and has now spread to be a commonly used term not only on the tubes, but in real life as well.
1. Holy shit The Dark was tits on a whale, man! (good)
1) An exclamation of surprise or shock, similiar to dayum.
2) A dismissal of something unhelpful or useless
1) A) I just won a new car!
B) Tits on a whale, that's sweet!
2) A) Hey guys, I can play "flight of the bumblebee" at 320bpm!
B) Yeah, except you've wasted your life learning a skill which is as useful as tits on a whale.