ichidan verb, intransitive verb
Archaic intransitive verb meaning 'to calm down' or 'to settle', especially of the stomach or mind. Most commonly found in the fixed expression 腹がいる (hara ga iru), meaning 'one's stomach settles' or 'one feels relieved'. In modern Japanese, 落ち着く (ochitsuku) is the usual word for calming down.
See also: 腹が居る
I finally feel relieved (lit. my stomach has settled).
この薬を飲めば腹がいるだろう。
If you take this medicine, your stomach will probably settle.