((肯定文で)) much[still] more;((否定文で)) much [still] less
弟子の作品があれほど立派なのだから,ましてやその師の作品はもっと立派だったに違いない
If the disciple's work is so splendid, his teacher's (work) must have been much more so.
彼女は言葉遣いも知らないし,まして行儀など知るはずがない
She does not even know how to speak properly, let alone how to behave./She doesn't even know polite language, 「much less [to say nothing of] good manners.