Platu means dumb, retarded and a bit crazy. All in one word.
If someone makes a mistake, you often say to him "Platu" as a sign that he is dumb,retarded and a bit crazy.
People often use it in CS:GO or League of Legends, but you can use it everywhere when the conditions are perfect.
A:You Platu!
B:Why i am Platu now?! "Name" is the Platu not me.
C:Sorry, my bad.