To itch ones genitalia.
When someones balls are itching this term is used to make a public statement that you are going to or are currently are itching them.
Comes from the totally awesome soundtrack of Scott Pilgrim vs. the World in the song "threshold" (played by the fictional band sex bob-omb originally written by Beck) at the part in the end when the lyrics go "reaching my threshold" but the word "reaching" is commonly mistaken for "itching"
dude 1: right so in 1777 george washington-
dude 2:DUDE!!! what are you doing??
dude 1:WHAT?? im itching my threshold!