na-adjective, noun
Used with a negative verb to mean 'not satisfactory', 'not decent', or 'not proper'. Often appears as ろくな〜ない or ろくに〜ない.
彼はろくな仕事をしない。
He doesn't do a decent job.
この町にはろくなレストランがない。
There isn't a single decent restaurant in this town.
ろくに勉強
There's no way you'll pass the exam without studying properly.
まとも means 'proper' or 'decent' in a positive sense, while ろく is almost always used negatively (ろくな〜ない).
The origin is uncertain; the kanji 碌 and 陸 are ateji (phonetic) spellings. The word is native Japanese and not derived from the kanji meanings.