42 is not the only answer!!!

Charles Arthur 'Y' got his start in crime at the age 18 when he stole $3.50 in pennies from a local post office, according to an issue of Time magazine, published 22 October 1922. Three years later he was arrested for a payroll robbery, September 16, 1925, in St. Louis, Missouri and served five years in prison. When paroled, 'Y' vowed that he would never see the inside of another prison. Entering into partnerships with more established criminals in the Kansas City underworld, he committed a series of bank robberies over the next several years; it was during this period that he earned the nickname 'X'. When the payroll master at one robbery first described the three perpetrators to the police, he referred to 'Y' as "a 'X' with apple cheeks". From then on he is known as 'XY'. 'XY' was also one of the prime suspects in the Kansas city massacre though he vehemently denied any involvement in it.

Id 'XY'.....:P