To be green means to appear young, inexperienced and foolish. To be cabbage looking means to be uneducated, plain and simple ( minded ) . So ''I'm not as green as I'm cabbage looking' means that you're not as naive and ignorant as you appear, or they seem to think that you are.
✍ Definition: Someone is not as naive or inexperienced as they appear 🤓 ✅ Example: Don’t try to fool her; she’s not as green as she is cabbage - looking.
