Encore

"When I come back like Jordan, wearing the 45. It ain't to play games with you, it's to aim at you. Probably maim you. If I owe you I'm blowin' you to smithereens, c*cksucker take one for your team"

Jay-Z "Encore"- The Black Album

Jay-Z has always toyed with the idea of retirement. He wants to go out on top, like Seinfeld. The only problem is he can't stop coming back because he loves rap too much. Michael Jordan was in a similar situation where he wanted to stop playing basketball before he got old and couldn't play at the level he was known for anymore. So he retired, but he came back. He had to wear the number 45 because he retired his old number 23, plus 45 was the number he originally liked and wore anyway (23 was half of 45. In his mind anyway)

But that is not all there is to that line. Jay-z is also referring to wearing the 45 as coming back with a gun or a pistol, like the street thug he used to be. This is illustrated by the line after there where he says "it's to aim at you, probably maim you".