A rather ironic misspelling of the word "stupid". It tends to take away the attention from the truly stupid party.
It can be used to justify that something is not stupid, simply because the person who used it does not know how to spell. Since a person in unable to spell it must mean that they are stupid, and thus have no idea what they are talking about. They are thought to be more stupid than the thing being criticized. (It does not matter at all that the keys "S" and "D" are located right next to each other on the keyboars)
The word can also be used doubly ironically, especially by hipsters
to show that something is, in fact, not stupid at all. It shows that anyone who would think something to be stupid must be stupid themselves.
Person A: "I think wars are so stupis, why can't people just learn to get along"
Person B: "Lol wut the heck is a stupis? rofl xD ur so stupid, Stupid. lololol"
Person A: "So you think that the mindless slaughter of innocent humans beings, for the ideology of the few in power, is an intelligent thing to do?
Person B: "lol stupis. What a loooser"
Person A "..."
Normal Person:"Ugh! Justin beiber is so stupis! His songs are so repetetive and shallow. Why does he have so many fans???"Belieber
: ur the stupid 1 xP u kant evan spall da werd stupid. lolololololol
Lol, breathing is so stupis.