Shlimazel is a Bogeyman who is new in town, and tries, rather foolishly, to frighten Susan in Biers during the events of Hogfather. Susan has considerable experience with Bogeymen and many other forms of monsters who attack children. She has developed methods of dealing with them, often using blunt instruments, but in this case her Death Voice: 'Go away and stop bothering me.' is sufficient to unseat and unnerve him and send him fleeing.
Later, Susan catches him under the bed of Twyla, but, since it's Hogswatch, sends him away with a "friendly" warning, not to come back - or else the next time it will be the blanket - and with fluffy bunnies on it! But there are no hard feelings - she tells him to try number nineteen; the governess there doesn't believe in bogeymen, but rather in algebra.
Shlimazel, appropriately enough, is a Roundworld Yiddish word meaning "a very unlucky or inept person who fails at everything" and is most famous to American audiences as the 2nd word of "Making Our Dreams Come True", the theme song to Laverne & Shirley