21/21/21 Share definition Flag A runner's challenge in which the athlete must run 21 miles and drink 21 beers within the 24 hours that constitute his or her 21st birthday. Alex tried to do a 21/21/21 for his birthday but got overzealous and passed out by noon. 21/21/21 by keykeys December 21, 2008 0 0 Get merch Get the 21/21/21 mug. Buy Now — $32.95 See more merch