An adverbial phrase meaning 'much less' or 'let alone'; used to introduce something that is even more true (often in negative constructions) than the preceding statement.
まだ歩けない赤ん坊に靴を履かせることは難しいし、ましてや一人で歩かせるなど無理だ。
It's hard enough to put shoes on a baby who can't yet walk; let alone make them walk alone.
彼は漢字も読めない。ましてや古典は理解できるはずがない。
He can't even read kanji. Much less could he understand classical literature.
今日は大雨で外出できない。ましてや山登りの計画は中止だ。
We can't go out today because of the heavy rain. Much less can we go hiking.