What is the meaning of 腹んばい and 腹んばいになって？
Also, is it read 「はらんばい」？
I understand that 腹んばい悪い seems to mean 'get mad' (腹が立つ), but this other usage seems like it isn't about anger.
Here is the context from Akutagawa's Nyotai:
楊某 {ようぼう} と云う支那人が、ある夏の夜、あまり蒸暑いのに眼がさめて、頬杖をつきながら腹んばいになって、とりとめのない妄想 {もうぞう} に耽っていると、ふと一匹の 虱 {しらみ} が寝床の 縁{ふち} を這っているのに気がついた。
From googling, this word still seems to be in use. I can't find this word in my kokugo dictionaries, which includes daijisen and genius (mac dictionary/goo). Why is that?
