A doomswitch is a hypothesised mechanism by which an online poker site will infer bad luck onto a player, causing him to lose large amounts of money. A player who has been doomswitched will miss many of his draws and suffer an inordinate number of bad beats and coolers.
Hypothesised reasons for obtaining a doomswitch on ones account include cashing out. Poker sites supposedly do not like money to leave the site, and hence the doomswitch may be invoked to discourage this.
The opposite of a doomswitch, where a player enjoys a prolonged period of good luck, is sometimes known as a "boomswitch".