When someone from another country comes into a country that they are foreign to and they see a white person, this may happen. The foreigner will then "thumb" the white. Or in other words they will stick their thumb up a ladies vagina or butt hole, or they will thumb a man's bum. Foreigners tend to be the kinkiest in bed, at least foreigners that are foreign to America, so this expression is often used a lot in gossip after hot, sexy foreign exchange students have hot, sexy parties... with Whites.
Marky J: So did you hear about Bin Laden and Britney?
Ashley Q: You mean how they were found in a cavetogether?
Marky J: Yeah, and FTW!
Ashley Q: :o