(Usually used with a negative) Not properly, not sufficiently; indicates something is not done well or adequately.
最近ろくに寝ていないので疲れが取れない。
I haven't been sleeping properly lately, so I can't get rid of my fatigue.
彼は朝ご飯をろくに食べずに出かけた。
He left without having a proper breakfast.
試験勉強をろくにしないで合格するのは難しい。
It's hard to pass the exam without studying properly.