When Eliahu HaNavi confronted the prophets of Baal and they couldn't deliver, he told them [1Kings 18:27]:
וַיְהִ֨י בַֽצָּהֳרַ֜יִם וַיְהַתֵּ֧ל בָּהֶ֣ם אֵלִיָּ֗הוּ וַיֹּ֙אמֶר֙ קִרְא֤וּ בְקוֹל־גָּדוֹל֙ כִּֽי־אֱלֹהִ֣ים ה֔וּא כִּ֣י שִׂ֧יחַ וְכִֽי־שִׂ֛יג ל֖וֹ וְכִֽי־דֶ֣רֶךְ ל֑וֹ אוּלַ֛י יָשֵׁ֥ן ה֖וּא וְיִקָֽץ׃
When noon came, Elijah mocked them, saying, “Shout louder! After all, he is a god. But he may be in conversation, he may be detained, or he may be on a journey, or perhaps he is asleep and will wake up.”
Rashi says that Baal's "journey" in question is a journey to the bathroom, to relieve himself. Indeed, some translations say that clearly. Most do not. What is the meaning of the Hebrew? Is there uncertainty? What is Rashi's source?