He prophesied and did not know what he prophesied, he prophesied and did not know what he prophesied.
He said something that became clear as a prophecy or prediction that came true. He said something to the point that the thing finally came true.
("And Pharaoh's daughter said to her, 'Take this child,'" said Rabbi Hama son of Rabbi Hanina: "She prophesies, but she does not know what she prophesies about." - Sota 12b, and so: "The land of Goshen - prophesies, but he does not know what he prophesies about: they will end up making it like a pond that has no fish in it." - Rashi, in the beginning of Ma 18, and why should I consider both of them?)
Translation
Expression (Hebrew)
הִתְנַבֵּא וְלֹא יָדַע מָה הִתְנַבֵּא, נִיבָּא וְאֵינוֹ יוֹדֵעַ מַה נִּיבָּא
Definition (Hebrew)
אָמַר דָּבָר שֶׁהִתְבָּרֵר כִּנְבוּאָה אוֹ תַּחֲזִית שֶׁהִתְמַמְּשָׁה. אָמַר דָּבָר לְתֻמּוֹ כְּשֶׁלְּבַסּוֹף הַדָּבָר הִתְגַּשֵּׁם.
("וַתֹּאמֶר לָהּ בַּת פַּרְעֹה הֵילִיכִי אֶת הַיֶּלֶד הַזֶּה- אָמַר רַבִּי חֵמָא בַּר' חֲנִינָא: מִתְנַבְּאָה וְאֵינָהּ יוֹדַעַת מָה מִתְנַבְּאָה"-סוֹטָה יב ע"ב, וְכֵן:"אֶרֶץ גֹּשֶׁן- נִבָּא וְאֵינוֹ יוֹדֵעַ מַה נִּבָּא: סוֹפָם לַעֲשׂוֹתָהּ כִּמְצוּלָה שֶׁאֵין בָּהּ דָּגִים"- רָשִׁ"י בְּרֵאשִׁית מָה יח, וְלָמָּה אֶשְׁכּוֹל שְׁנֵיכֶם)