When a person takes a word or phrase from what you just said, and uses it in a completely unrelated, usually tedious and annoying context. "Wordjackers" think they are actually engaged in dialogue, when in fact their behavior demands immediate physical punishment.
Shawna: Did you see Christian Bale in The Machinist?
Trixie: No but my unemployed stepdad is a machinist!
Shawna: Don't wordjack me! (kicks Trixie in the face)