An abbreviation of two pilots from the
popular japanese
anime Gundam Wing, heero and duo. It is mostly used in yaoi fanfiction and fanart by fans for the romance between the characters. There are various of these pairings, for starters, the meanings:
1 > heero 'reference for leader of the gundam pilots'
2 > duo 'means two in italian (i think)
3 > trowa 'trois = three in french'
4 > quatre 'means four in french'
5 > wufei 'if you say this name, you can hear
five somehow'
6 > zechs 'means six in
german'
13 > treize 'means thirteen in french'
the most common pairings are 1x2, 1x2x5, 3x4, 6x13, 13x5