noun
on one's back; face up
Describes a posture of lying or falling backward. Most often used adverbially as のけざまに.
彼はのけざまに倒れた。
He fell on his back.
のけざまに寝転んで空を見上げた。
I lay on my back and looked up at the sky.
あおむけ is the common word for 'face up' or 'on one's back'; のけざま is a less common synonym with a similar meaning.
The derivation is uncertain. The kanji 仰け様 suggests a connection to 仰ぐ (to look up) and 様 (manner), but the word is now almost always written in kana.