expression
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...'.
彼は英語はおろか、日本語もあまり話せない。
He can't speak much Japanese, let alone English.
その村には電気はおろか
The village doesn't have running water, not to mention electricity.
na-adjective
Archaic adjectival noun meaning neglectful or careless. Now largely replaced by おろそか, which is the modern standard form.
See also: 疎か (おろそか)
古い文献では「おろかなる行い」のように使われることがある。
In old texts, it is sometimes used like 'negligent conduct'.
Modern standard word for 'negligent' or 'careless'. おろか in this sense is archaic and rarely used today.
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.