Wiki says -
'Opera singers use "Toi Toi Toi", an idiom used to ward off a spell or hex, often accompanied by knocking on wood, and onomatopoeic, spitting (or imitating the sound of spitting). Saliva traditionally had demon-banishing powers. From Rotwelsch tof, from Yiddish tov ("good", derived from the Hebrew טוב and with phonetic similarities to the Old German word for "Devil.")[21] One explanation sees "toi toi toi" as the onomatopoeic rendition of spitting three times. Spitting three times over someone's head or shoulder is a gesture to ward off evil spirits. A similar-sounding expression for verbal spitting occurs in modern Hebrew as "Tfu, tfu" (here, only twice), which some say that Hebrew-speakers borrowed from Russian.[22]
An alternate operatic good luck charm, originating from Italy, is the phrase "in bocca al lupo!" ("In the mouth of the wolf") with the response "Crepi il lupo!" ("May the wolf die").