"Ita" (痛) refers to "pain" in Japanese, and the cars covered in geeky stickers were originally called "ita-sha" or "painmobiles" due to the pain they inflict on the owners' wallets and the pain they inflict on everyone's eyes. Since then "ita" has come to mean anything that's covered in nerdy images.
Called "ita tights" ("painful tights"), these are tights with anime or game characters drawn on them. There's a walk through below in Japanese, but the pictures should hopefully clue you in if you don't know the lingo.
Word to the wise: use white or silver tights, as they can easily show off the drawings and are easier to trace with.