Shulchan Aruch Orach Chayim, Siman 3, Seif 1 says; כְּשֶׁיִּכָּנֵס לְבֵית הַכִּסֵא יֹאמְרוּ הִתְכַּבְּדוּ מְכֻבָּדִים וְכו'. וְעַכְשָׁו לֹא נָהֲגוּ לְאָמְרוֹ
When one enters into the throne room (bathroom stall), say: "התכבדו מכובדים, etc." But now we are not habituated to recite it.
What is the full phrase? Where is it from? What does it mean? Why don't we say it anymore? And should we say it? I thought we weren't allowed to speak in a bathroom.