1. What does it mean?
꼰대 is associated with an overbearing, condescending attitude, often expressed through seniority or 'in my day' lectures. It can criticize refusing to listen while demanding that others follow one's own way.
Also written or said as 꼰머, kkondae
2. The nuance
It is an insult, not a synonym for every older person. A young person can act this way too; identifying the specific conduct is fairer than stereotyping an age group.
3. Examples
“나이가 많다는 이유로 전부 꼰대라고 부르면 안 되지.”
You shouldn't call everyone a condescending old-timer just because they're older.
“내 경험만 정답처럼 말한 건 꼰대 같은 태도였다고 돌아봤어.”
I reflected that presenting only my experience as the right answer had been overbearing.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.