someone who isn't quite alcoholic, nor are they drinking sensibly. Most likely someone is high-functioning in society, but drinks more then they should, but doesn't feel the need to drink in the morning in the afternoon. a semi-alcoholic is most likely a student at university or weird spooky kid who doesn't have much direction in their life.
There most likely to drink out of sheer boredom rather then addiction because sober reality is bullshit, but maybe they have taste and prefer expensive fancy german beers.
"I drink almost every night or other night, but I have a girlfriend, keep two part time jobs and I'm on track for a 2:1 degree" - a semi-alcoholic.