When a man cums in a girls mouth after head, he swishes his "meat" (penis) around in her mouth while the cum is still unswallowed or not spat out in a rotational circle as the blades of a blender would do, causing a meat smoothie.
A meat smoothing is literally when you mix the smoothie (cum) with your meat (penis).
Girl: I was with my boyfriend last night and he gave me a meat smoothie. I could taste everything, it was fantastic.
Guy: I'm going to put my meat in your mouth and make a smoothie with my juices.