still more; even more; let alone
Used in affirmative contexts to emphasize that something applies even more strongly to a second case. Often follows a clause or noun marked with は or も.
大人でも難しいのに、まして子供には無理だ。
It's difficult even for adults, let alone children.
It's tough even on a normal day, and even more so today with the staff shortage.
still less; much less
Used with a negative verb to mean that something is even less likely or possible. Often follows a negative statement and introduces a stronger negative case.
He can't even do simple calculations, much less solve complex problems.
I can't even speak Japanese, so there's no way I can speak English, still less.