עַ֤ל הַר־גָּבֹ֙הַּ֙ וְנִשָּׂ֔א שַׂ֖מְתְּ מִשְׁכָּבֵ֑ךְ גַּם־שָׁ֥ם עָלִ֖ית לִזְבֹּ֥חַ זָֽבַח׃
Upon a high and lofty mountain Hast thou set thy bed; Thither also wentest thou up To offer sacrifice.
Rashi on Isaiah
you placed your couch The couch of your adultery to idolatry on the high mountains.
Ibn Ezra on Isaiah
Upon a lofty and high mountain, etc. Like a harlot that commits adultery in an open place, that is, publicly.