much less; let alone; to say nothing of
Used to emphasize that if something is true for a stronger case, it is even more true for a weaker one. Often follows a negative or hypothetical statement.
He can't even read simple kanji, much less a newspaper.
I'm so busy I don't even have time for lunch, let alone a break.
言うまでもなく means 'needless to say' and stands alone, while ましてや connects two clauses to show that a claim applies even more strongly to a second case.
Derived from the adverb まして (much more, still less) with the emphatic particle や added for reinforcement.