A person who drinks a lot of alcohol; a tippler or heavy drinker (colloquial, often affectionate).
彼は若いころからの飲ん兵衛で、週に何度も居酒屋に行く。
He's been a heavy drinker since he was young and goes to izakayas several times a week.
飲ん兵衛同士だと、話がすぐに盛り上がる。
When two drinkers meet, the conversation quickly gets lively.
母は自分のことを『ちょっとした飲んべえよ』と笑って言った。
My mother laughed and said, 'I'm kind of a bit of a drinker.'