This trophy is for all the haters who were sleeping on me!
|Cash me outside||Come outside and I'll beat you|
|Torch||To beat someone in competition|
|FTW||For the win|
|FTEW||For the epic win|
When you underestimate or undervalue someone or something, you are "sleeping on" them. The phrase plays off the idea that a person cares so little about something to the point where he is literally sleeping.
People may use the phrase in various contexts to lament how someone didn't give you or something else the proper attention it deserves. It may be an employer who wrote off an applicant based on their appearance or a girl who won't give a guy a chance because he's too nice.
It is also commonly used in the realm of competition when a sports team or individual competitor overlooks a weaker opponent. This plays into the "David and Goliath" trope.