Mami's knocked up cause she wasn't watched over
Knocked down by some clown when child support knocked
No he's not around
In the first line, he’s using “mommy” or “mami” as he often does for a general term for a female. In this case probably a young teen. Saying that the reason she was impregnated or “knocked up” is because her parents didn’t watch over her (or couldn’t), and were not around (i.e. mom working lots of hours, father absent or incarcerated). So as a result the young lady had sex behind her parents' backs.
“Knocked down” after being “knocked up” because getting pregnant was a misfortune rather than a joyous event as it is in middle-class families. The mother is left to raise the child by herself like many other inner-city women
This section can also be a reflection on Jay’s childhood. His father (who he says numerous times he doesn’t want to be like) knocked up his mother because she didn’t have someone looking out for her. That clown reference is to his father who left Jay as a child with his mother to raise by herself with no child support payments.
Sidenote: Jay-Z in Decoded explains a part of this line saying; “Again I’m talking about the flipside of the Money, Cash, Hoes” type songs. Sex can knock you up and knock you down."
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