paying out of one's own pocket; paying one's own way
Used when someone covers an expense personally, especially when it might normally be paid by an employer, group, or other party. Often appears in phrases like 自腹を切る (to foot the bill oneself).
I paid the transportation costs out of my own pocket.
会社の飲み会なのに、上司が自腹を切ってくれた。
Even though it was a company drinking party, the boss footed the bill himself.
自腹 is a casual, colloquial way to say 'out of one's own pocket', while 自費 is the neutral/formal term used in official or written contexts.
Compound of 自 (self) and 腹 (belly), literally 'one's own belly'. The figurative use of 腹 for money or resources is an old metaphor in Japanese, similar to 'out of one's own pocket' in English.