7-7 or 7/7
the 7th of July 2005, the day on which a series of suicide bombings were carried out on the London public transport system by Islamic fundamentalists
the seven deadly sinsRead more in this article about some frequently asked questions and fun facts related to our definitions.
Word Origin for seven-seven
C21: by analogy with nine-eleven
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012