To be like a vegetable, one who has no brain nor cognitive thinking patterns.
To procrastinate or be lazy.
To chill out like fat people.
Instead of going to class, Jason "Cabbaged" all day.
Or
Beth didn't give the guy a working number because of the stains on his shirt and he looked like a "cabbage".
Commonly its someone who views any form of education as a "waste of time" and usually tries to become a professional at a sport but ends up getting a "good effort" award, they also enjoy spending time fantasizing that they will earn millions in the future.
Unfortunately these types of people are often delusional, see themselves above everyome else and won't listen to anyone else's opinion apart from another cabbage. So it's hard to persuade them to change.
In the end, a cabbage will become a gym teacher, or a coach for small team which won't achieve much but more "good effort" awards and barely getting minimal wage.
That guys a cabbage.
How come?
His grades are falling and his sports career doesn't exist, but he keeps saying that he's better then everyone else.