adverb
much more; let alone; not to mention
Formal, somewhat literary adverb used to emphasize that if something is true for a lesser case, it is even more true for a greater case. Often follows a negative statement or a rhetorical question, and is typically paired with ～さえ or ～すら. In modern Japanese, ましてや is more common in everyday speech.
大人でさえ難しいのに、いわんや子供においてをやだ。
It's difficult even for adults, let alone children.
彼は簡単な計算もできない。いわんや複雑な方程式をやだ。
He can't even do simple calculations, much less complex equations.
Derived from classical Japanese 言はんや (iwanya), literally 'needless to say', from 言ふ (iu, 'to say') + ん (mu, volitional/intent auxiliary) + や (ya, rhetorical question particle). The modern form いわんや is a fossilized expression.