A metaphor for ejaculation into the pussy, ass, or mouth. The cock is the "needle" and the cum is the serum. The cock penetrates and "injects" cum like a needle injects serum into the body after piercing the skin.
An expression of the desire to fuck or be sucked off and then cum inside the fuck partner.
She'll know you're a man after you give her your hot beef injection!! (heard at summer camp).
Open your mouth so I can give you my hot beef injection.