/le-tərbäm/
n. A piece of email containing live data intended to do nefarious things to the recipient's machine or terminal. It is possible, for example, to send letterbombs that will lock up some specific kinds of terminals when they are viewed, so thoroughly that the user must cycle power to unwedge them. Under UNIX, a letterbomb can also try to get part of its contents interpreted as a shell command to the mailer. The results of this could range from silly to tragic.
See also Trojan horse; compare nastygram.