Pump, Johnny Johnny
A “Johnny pump” is NY slang for an open fire hydrant that kids are splashing around in front of on a hot summer day in the ghetto
The “Johnny Johnny” refers to that frustrating-ass game that the masochistic children would play in elementary school involving stabbing between your fingers with a sharp object while saying “Johnny Johnny whoop! Johnny whoop! Johnny Johnny Johnny” etc.
Since he intends to “stab” this girl (sexually) and then “hose her down” (also sexually) both of these probably refer to sexual things
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