The shovel speech (at least the one I'm thinking of; I'm sure there are others) is a speech where a person's family or friend says, to the person's boy/girlfriend:

(I'm paraphrasing here because I can't remember it exactly)

Family Member/Friend: *holds out a shovel* "This is a shovel. If you ever even think of harming them, I will use this shovel to beat you to death and then I will use it to bury you."

That's more or less what it is. The ones I remember reading are so much better but I don't feel like hunting for one.