To attack someone, usually in a frat-style. A group bear hug that is not condoned by the hugged party. Also a group-beating in a friendly fashion (e.g. mercilous smack-down with pillows).
Origins of giving someone a count as to when the attack will commence ("We're giving you to three! Unos-Dos-Tres!")
Alex: "Let's DosTres the new kids."
Adam: "DosTres'ing them is a fantasic bloody idea."
Nick: (grunt like caveman).