Anybody and anything can be a "jack". Take a jacklight, though, to the highway in the night hunting for bootleggers' booze, and you're a "hijacker".
Take a jacklight, now, to a lonesome computer in the night hunting for other senders' threads, and you're — not a "cojacker" but a "threadjacker":
›› Threadjacking means to take over an email conversation. Find threadjacking defined here.