My pen'll go off when I half-cock it

from Eminem – Rap God Lyrics on Rap Genius

Meaning

Half-cock is form of safety used on revolvers and lever-actions rifles, it doesn’t allow the hammer to rest on the percussion cap, so it will not fall if the trigger is pulled. It is one of the most effective forms of safety on guns, but Em is still so wild and out of control that he will end up firing off anyway.

When you click a pen into the writing position you “cock” the end and it then rests at “half” the height. So when he “half-cocks” his pen it’ll “go off” by writing bars, perhaps even these. The other is a play on the words “pen” and “cock” and to “go off” meaning ejaculate.

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