The Rama (Rabbi Moshe Isserles of Cracow, 1525-1572), in his glosses to the Shulhan Aruch (Yoreh De’a 376), mentions the Halacha that one should not "open his mouth to the Satan." This means that one should not speak about events that he does not wish to transpire, such as disasters and catastrophes, as words have the power to cause these misfortunes to happen. The Sages teach, "Berit Keruta La’sfatayim" – there is a "covenant" made with the lips whereby they have power to cause that which they speak about. The word "Dibbur" ("speech") is derived from word "Dabar" ("thing"), which generally refers to tangible objects. Speech has substance and force, and therefore it must be used with great caution.
Thus, for example, if a person has not heard from somebody in a long time, and that other person has not returned his phone calls or letters, he should not say, "I haven’t heard from him; he must have died." The mere utterance of these words could cause the misfortune to happen, and thus these words should not be said. Likewise, if somebody is reporting to his fellow about his misfortune or distressing situation, he should preface his remarks by saying, "Lo Alechem" ("This shall not befall you"). For example, if a person is reporting that he is sick, he should first say "Lo Alechem" to make it clear that he does not wish this misfortune to befall his fellow.
A person should not curse himself, or curse somebody else, as the curse could come back to hurt him, God forbid. If a person is recalling to his fellow how he cursed an anti-Semitic gentile, he should speak in the third person form – "I cursed him that such-and-such will happen to him" – rather than saying, "I said: I wish such-and-such will happen to you" – as this would sound as though he is placing the curse on his fellow.
The Gemara in Masechet Shabbat (62b) lists several things that could potentially cause poverty, one of which is a woman’s cursing her husband because he does not buy her jewelry. This demonstrates that even if a person has a legitimate grievance against somebody – such as against one’s spouse – he must not curse that person or express his wish that he should suffer misfortune.
Summary: One should not speak about misfortunes that he does not wish to experience, or utter a curse, as the mere utterance of the words could cause those unfortunate events to transpire.
