To inject an illicit substance (heroin, et. al.) directly under the skin rather than in the muscle, or the preferred way, in the vein. The result is typically, a small bump. This is typically done because your needle is crappy or your veins are shot. Shooting in the muscle typically hurts, so bumping isn't a bad way to go, and it beats snorting, arms down.
I had a shitty needle and needed to bump
My veins were all fucked up and I needed to bump