Tired as hell one night, with my sleep deprived brain trying to figure out a good gamertag. My usual usernames were taken, and I thought "ah man, mandalorian slayer would be so cool!" But it didn't fit. Thus mandoslayer1153 was unfortunately birthed into the world. As for the 1153, I've used that in literally every username, since when I was making my first runescape account, I tried naming my character pikachu, which was of course taken, and they gave the suggestion of pikachu1153. And the 1153 has stuck ever since.