Military colloquialism for "re-enlisting". Many services offer large cash payments to enlisted men and NCOs who re-up, especially if they've been trained in critical skills. The re-up bonus maxes out at about $20,000 if the economy is red hot and they have a hard time keeping grunts. However a radar/communications specialist might get a $30,000 re-up bonus and a whopping $40,000 might be dangled before the eyes of a Korean language specialist. Other perks for those re-upping allow some flexibility where they want to be based (say Italy instead of Kentucky this time around...).