The Kaf Ha'haim (Halachic work by Rabbi Yaakov Haim Sofer, 1870-1939) writes (Orah Haim 250:12) that it is forbidden to mention the word "Shabbat" in places where Halacha forbids speaking words of Torah, such as in restrooms, a Mikveh, and places with filth. The reason, he explains, is that, as the Zohar comments, the word "Shabbat" is one of the Names of the Almighty, and as such the very mention of the word must reflect the honor owed to the Name of God.
This concept, of "Shabbat" constituting one of the divine Names, helps us understand an otherwise perplexing passage in the Talmud. The Gemara in Masechet Megila tells that when Haman sought to malign the Jewish people to Ahashverosh, he referred to them as "Shehi Pehi." Rashi, in his commentary, explains these words as acronyms representing the phrases "Shabbat Hayom" ("It is Shabbat today") and "Pesah Hayom" ("It is Pesah today"). Haman described the Jews as a lazy people, who found excuses for their inactivity by inventing occasions such as Shabbat and Pesah. The question immediately arises, why did Haman employ such a subtle term in reference to what he perceived as the Jewish nation's indolence? Why didn't he just explicitly say that the Jews observe too many days of rest?
The answer becomes abundantly clear once we realize that the word "Shabbat" is one of the Names of God. Even Haman recognized the significance and sanctity of this word, and thus out of his sheer repugnance for the Jewish people and their faith, he refused to even utter the word "Shabbat." He instead invoked an acronym to convey his venomous hatred of the Jews, to avoid saying a term associated with holiness.
Summary: One should not say the word "Shabbat" while in the restroom, Mikveh, or other places where it is inappropriate to speak of Torah matters.
