吃老本 is a colloquial set phrase meaning 'to rest on one's laurels' or 'to live off past achievements without making new progress'. The literal image is 'eating one's old capital' — consuming what you've already earned without replenishing it. It's often used in work, study, or career contexts to criticize someone who stops improving and relies on past success. The phrase is a separable verb (离合词): you can insert measure words or modifiers between 吃 and 老本, like 吃了三年老本 ('lived off past achievements for three years').