וְחַלְלֵיהֶ֣ם יֻשְׁלָ֔כוּ וּפִגְרֵיהֶ֖ם יַעֲלֶ֣ה בָאְשָׁ֑ם וְנָמַ֥סּוּ הָרִ֖ים מִדָּמָֽם׃
Their slain also shall be cast out, And the stench of their carcasses shall come up, And the mountains shall be melted with their blood.
Ibn Ezra on Isaiah
באשם Their stink. It is a noun derived from באש after the form of קׂדֶשׁ holiness.