not to mention; let alone; much less
Used after 〜は or 〜も to emphasize that something is far beyond what was just mentioned. Often implies 'it goes without saying that...' or 'not only that, but even...'.
He can't speak much Japanese, let alone English.
The village doesn't have running water, not to mention electricity.
Archaic adjectival noun meaning neglectful or careless. Now largely replaced by おろそか, which is the modern standard form.
See also: おろそか
In old texts, it is sometimes used like 'negligent conduct'.
おろそか is the modern standard word for 'negligent/careless', while 疎か（おろか）in that sense is archaic and now rarely used.
まして and 疎か（おろか）both build an escalating 'not to mention, even more so' comparison, but まして works as a standalone connector while 疎か requires 〜は/〜も before it.
もちろん states plainly that something is 'of course' true, whereas 疎か（おろか）specifically contrasts a lesser case with a greater one via 〜はおろか〜も.
疎か read as おろか means 'let alone' (used in 〜はおろか), a completely different word from おろそか, which means negligent or careless, despite sharing the same kanji.
Usually written in kana; the kanji spelling 疎か is rare and may be confused with おろそか.
Kanji spelling is uncommon; learners should prefer the kana form.
Derived from classical Japanese. The adjectival noun sense is related to おろそか, but the expression sense developed separately as a grammaticalized pattern.