1.
turning over (in bed)
Rolling from one position to another while sleeping.
I rolled over many times in the middle of the night and couldn't sleep well.
2.
betrayal; defection
Switching sides or betraying one's allies.
寝返り has both a literal physical meaning and a figurative meaning of betrayal.
Physical meaning
Rolling over in bed while sleeping. An important developmental milestone for babies (赤ちゃんの寝返り).
Figurative meaning
Betray, defect, switch sides. The image is of "turning over" one's allegiance.
Common patterns
- 寝返りを打つ: to roll over (physical) / to defect (figurative)
- 寝返りができる: able to roll over (baby milestone)
- 敵に寝返る: to defect to the enemy