what is the meaning of 腹んばいになって?

Question

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?

Answer 1

[腹ん這い]{はらんばい}になって is an 音変化{おんへんか} (shift in sound) of [腹這い]{はらばい}になって "flat on one's belly".

In a similar way, [四つん這い]{よつんばい} is an 音変化 of [四つ這い]{よつばい} for example.

See also 腹這い.