まして（や）
mashite (ya)
let alone; much less; all the more
[Let alone / Much less / All the more]
Connection: Statement A。まして（や）B
Usage: After stating that A is the case, emphasizes that B (a more extreme case) is even more so. Can be used in both positive and negative directions. "If A is true, then B is even more so / even less likely."
Examples:
大人でも難しい。まして子供にできるわけがない。(It's difficult even for adults. Let alone children — there's no way they can do it.)
日本語は難しい。ましてや敬語はもっと難しい。(Japanese is difficult. Much more so for keigo [honorific language].)
平日でさえ混んでいる。まして休日はもっとひどい。(It's crowded even on weekdays. Much more so on holidays.)