When you and your friend stick your thumbs up your own asses and then you grab a hold of each others fists and work them up and down. (Similar to a dutch rudder but with thumbs and asses instead of dicks)
Me: "Jem and I gave each other a Dutch Thumber yesterday."
X-Ray: "Is that gay?"
Vav: "Of course its not gay."
X-Ray: "Is that gay?"
Vav: "Of course its not gay."
by The Thumbing Dutchman February 5, 2013