וְעַתָּה֙ הִתְעָ֣רֶב נָ֔א אֶת־אֲדֹנִ֖י הַמֶּ֣לֶךְ אַשּׁ֑וּר וְאֶתְּנָ֤ה לְךָ֙ אַלְפַּ֣יִם סוּסִ֔ים אִם־תּוּכַ֕ל לָ֥תֶת לְךָ֖ רֹכְבִ֥ים עֲלֵיהֶֽם׃
Rashi on Isaiah
wager now on this matter, if you will be able to accomplish it. This is an expression of a wager, which is called gajjer in O.F.