It has to do with the Only Patrick Martin Stump!!
Singer of Fall Out Boy!!!
You use it when something has lacks creativity, or music , or coolness<3
And it needsss it!!
"This is a sad sack. You look like a bunch of retards humping a doorknob!! You need to Stump it up!"
It's something a girl says to a guy to make him shut up.
To explain to a guy that it's something a girl does
guy:I don't get why girls are like that.
girl:you just wouldn't know
guy:You spent $100 on cds
girl: you just wouldn't know