Meaning "shit is weak", this phrase is appropriate when a person, object, or event is weak, not as cool as originally thought, or otherwise lacking.
Examples of when it is appropriate to use this phrase:
Your friend posts a video of a rice rocket on Facebook, exclaiming how cool it is, but it isn't. You comment "Shit's weak."
You buy some espresso from Starbucks, only to find that it isn't as delicious as everyone says. You mumble, "Shit's weak."
You go to a party only to find that most of the attendees are not of the opposite sex (or not of the same sex, whatever floats your boat). "Shit's weak."
How to described something that is more lame than lame can describe.
Aw, man, you slept with that ugly bitch? Shit's weak.
Originally taken from the movie Grandma's Boy. To define someone's act, work, game, comeback or current doing to be weak.
<In the proccess of an argument>
Rob: "...Not as bad as your face" <- (Bad comeback)
Stu: "Shit's weak!"
(Rob gets sniped)
Stu: "AAAAAwwH! Shit's weak, man"