noun
dozing off while lying down; falling asleep unintentionally
Refers to dozing off or falling asleep without intending to, often while lying down or in a relaxed position. The kanji 転び (rolling/falling) suggests the accidental nature of the sleep.
ソファで転び寝してしまった。
I accidentally dozed off on the sofa.
転び寝は体に良くないと言われる。
They say that dozing off unintentionally is not good for your health.
Compound of 転び (korobi, 'falling' or 'rolling', from 転ぶ korobu) and 寝 (ne, 'sleep'). The combination suggests falling into sleep unintentionally.