expression, ichidan verb
resolve; make up one's mind; decide
Idiomatic expression meaning to firmly decide or resolve to do something, often after hesitation. Literally 'to settle one's stomach'.
彼は腹を決めて、会社を辞めることにした。
He made up his mind and decided to quit the company.
いつまでも迷っていないで、そろそろ腹を決めなさい。
Stop hesitating forever and make a decision already.